compile_fail_utils: Verify path exists

[MrGVSV]

Objective

It looks like running compile_fail_utils::test on an invalid path just skips the test. I'm not sure why ui_test doesn't fail the test, but it seems pretty easy to accidentally cause a test to be skipped (I experienced this while doing some refactoring on bevy_reflect).

Solution

Check to make sure the given path exists before continuing on with the tests.

Alternatively, we could look into seeing why this doesn't work properly upstream. But I figured this solution was simple enough just to implement directly without having to worry about updating ui_test.

Testing

To verify that this works as expected cd into crates/bevy_reflect/compile_fail. Then run the following:

cargo test --target-dir ../../../target

All compile fail tests should pass. Now edit the path used in crates/bevy_reflect/compile_fail/tests/derive.rs. For example:

fn main() -> compile_fail_utils::ui_test::Result<()> {
-    compile_fail_utils::test("reflect_derive", "tests/reflect_derive")
+    compile_fail_utils::test("reflect_derive", "tests/does_not_exist")
}

Run the tests again:

cargo test --target-dir ../../../target

Verify the test fails with an error like:

Error: path does not exist: "tests/does_not_exist"

[TrialDragon]

LGTM