FR: allow loading phase failures to be caught in Starlark tests

Currently Skylib has [loadingtest.equals](https://github.com/bazelbuild/bazel-skylib/blob/main/lib/unittest.bzl#L636) to verify that a macro has expected output. However, we have no way to verify that a macro *fails* as expected: a loading phase failure cannot be caught, it causes the build to fail. Thus there is no loading phase analogue of `analysistest.make(..., expect_failure = True)`.

It would be useful if we had a mechanism similar to `--allow_analysis_failures` to make it possible to write a Starlark test framework that can catch and verify loading phase tests.

Potentially interested parties: @murali42