Add UI tests for existing lints

[BD103]

Part of #31.

This is a continuation of #125 that actually adds UI tests for all of the current lints. It was split off to make #125 easier to review.

To test this, run:

cargo test -p bevy_lint --test ui

To bless changes, run:

cargo test -p bevy_lint --test ui -- --bless

There are a few additional options available if you replace --bless with --help, too.

[BD103]

You'll probably notice the large file count when reviewing this PR. This is due to a multitude of reasons:

1. Seeing a wall of green checkmarks brings me happiness.
2. I like testing every single aspect of my code, even the bugs, to ensure I always know when something changes.
3. Certain tests are required to be split into multiple files, such as main_return_without_appexit where you cannot have multiple fn main() in the same file. (Other examples include //@check-pass comments, which ensure no warnings are emitted for a file.)
4. For each test that emits warnings / errors, a .stderr file is emitted with the plain-text content of the warning. If these diagnostics contain machine-applicable fixes, a .fixed file is also emitted by rustfix.

If you ever have trouble understanding a test, it's usually my fault and not yours. Please let me know if that's the case! You can also consult ui_test's README and examples.

[BD103]

With that all out of the way, I've finished writing tests for all of our lints! This is ready for review, once #125 is merged. :)

[BD103]

#125 is merged, so this is ready for review now!

[TimJentzsch]

Really nice to have tests for these.

Left some minor comments, but feel free to merge once you think it's ready.