Improve check_mocks Makefile target

The existing check_mocks target isn't helpful in determining why a diff happens on the server and not on the client. It only compares the names of changed files instead of the actual contents. This commit changes the behaviour so that it guarantees that the existing commit matches the regenerated mocks.

There is also an issue where my machine will generate mocks grouped inside a single import statement, whereas the server generates a statement per-import. This MR will try to fix that.

Fixes #45 (closed) and gitlab-org/gitlab-runner#25188 (closed)