_yet another_ bugfix to the version check X-(

When I stopped running the version check before the tests I also stopped
initializing Version, which can be used in tests to watch out for font
changes across versions. As a result I started seeing a test failure
with LÖVE v12.

It looks like all manual tests pass now. And we're also printing the
warning about version checks before running tests, which can come in
handy if a new version ever causes test failures. The only thing that
makes me unhappy is the fact that we're calling the version check twice.
And oh, the fact that this part around initialization and version
management is clearly still immature.

I'll capture some desires and fragmentary thought processes around them:

* If there's an error, go to the source editor.

* But oh, don't go to source editor on some unactionable errors, so we
  include a new `Current_app` mode for them:
  * Unsupported version requires an expert. Just muddle through if you
    can or give a warning someone can send me.
  * A failing test might be spurious depending on the platform and font
    rendering scheme. So again just provide a warning someone can send
    me.

  [Source editor can be confusing for errors. Also an editor! But not
  showing the file you asked for!]

* But our framework clears the warning after running tests:
  * If someone is deep in developing a new feature and quits -> restore
    back in the source editor.

  [Perhaps `Current_app` is the wrong place for this third hacky mode,
  since we actually want to continue running. Perhaps it's orthogonal to
  `Current_app`.]

  [Ideally I wouldn't run the tests after the version check. I'd pause,
  wait for a key and then resume tests? "Muddle through" is a pain to
  orchestrate.]

* We store `Current_app` in settings. But we don't really intend to
  persist a `Current_app` of 'error'. Only the main app or 'source'
  editor.

  [Another vote against storing 'error' in `Current_app`.]

* So we need to rerun the version check after running tests to actually
  show the warning.

  [Perhaps I need to separate out the side-effect of setting `Version`
  from the side-effect of changing `Current_app`. But that's not right
  either, because I do still want to raise an error message if the
  version check fails before running tests. Which brings us back to
  wanting to run the tests after raising the version check..]

One good thing: none of the bugs so far have been about silently
ignoring test failures. I thought that might be the case for a bit,
which was unnerving.

I grew similar muddiness in Mu's bootstrap system over time, with
several surrounding modes around the core program that interacted poorly
or at least unsatisfyingly with each other. On one level it just feels
like this outer layer reflects muddy constraints in the real world. But
perhaps there's some skill I still need to learn here..

Why am I even displaying this error if we're going to try to muddle
through anyway? In (vain) hopes that someone will send me that
information. It's not terribly actionable even to me. But it's really
intended for when making changes. If a test fails then, you want to
know.

The code would be cleaner if I just threw an unrecoverable error from
the version check. Historically, the way I arrived at this solution was:
  * I used the default love.errorhandler for a while
  * I added xpcall and error recovery, but now I have situations where I
    would rather fall back on love.errorhandler. How to tell xpcall
    that?
But no, this whole line of thought is wrong. LÖVE has a precedent for
trying to muddle through on an unexpected version. And spurious test
failures don't merit a hard crash. There's some irreducible requirement
here. No point making the code simplistic when the world is complex.

Perhaps I should stop caching Version and just recompute it each time.
It's only used once so far, hardly seems worth the global.

We have two bits of irreducible complexity here:
  * If tests fail it might be a real failure, or it might not.
  * Even if it's an unexpected version, everything might be fine.
And the major remaining problem happens at the intersection of these two
bits. What if we get an unexpected version with some difference that
causes tests to fail? But this is a hypothetical and not worth thinking
about since I'll update the app fairly quickly in response to new
versions.

Manual_tests.md

@@ -18,6 +18,7 @@ Initializing settings:
   - no log file; switching to source works
 
   - run with an unsupported version. Error message pops up and waits for a key. The app attempts to continue, and doesn't receive the key.
+  - run with a LÖVE v12 release candidate. No errors; it is a supported version. All tests pass.
 
 Code loading:
 * run love with directory; text editor runs

app.lua

@@ -8,6 +8,7 @@
 --     and a source editor, while giving each the illusion of complete
 --     control.
 function love.run()
+  App.version_check()
   App.snapshot_love()
   -- Tests always run at the start.
   App.run_tests_and_initialize()