Added unit test case for path_exists

testcases/003-path-exists/expected_output.txt

@@ -0,0 +1 @@
+EXISTING: yes | NONEXISTANT: no

testcases/003-path-exists/i3status.conf

@@ -0,0 +1,14 @@
+general {
+        output_format = "none"
+}
+
+order += "path_exists EXISTING"
+order += "path_exists NONEXISTANT"
+
+path_exists EXISTING {
+        path = "testcases/003-path-exists"
+}
+
+path_exists NONEXISTANT {
+        path = "testcases/003-path-exists/nonexistant"
+}