This test had previously (before #231) been making a change to a file in
the kaniko dir, then checking that it isn't being snapshotted. This was
to test the whitelisting logic, which makes sure that changes to /kaniko
aren't included in images. However the test creates a temporary dir, so
the kaniko dir is actually in /tmp/<some temp dir>/kaniko, and
in #231 the logic was simplified to no longer have a special case for
tests. The test continued to pass because `MaybeAdd` noticed that the
kaniko file wasn't changing, and didn't add it. After changing this to
always add the files, it revealed that this was left behind by accident.
I also opened #307 to add integration test coverage for this logic.
I also marked `CheckErrorAndDeepEqual` as a helper function so that when
it fails, the line number reported is where that was called.
2fe93f2911