Expand tests

Besides expanding test coverage, this moves the tests from
`bump-formula-pr-throttle_spec.rb` into `bump-formula-pr_spec.rb`, as
test files should directly correspond to a related source file. The
latter uses `typed: strict` but this results in a lot of type errors
when adding tests, so we have to drop it back to `typed: false` like
other test files that have the same errors.