[Feature] Support `expect.assertions`

[mrmckeb]

I saw that the this was originally available, but not working, and was later removed:

• [BUG] expect.assertions() and expect.hasAssertions() Not Working #11845
• [BUG] Hide unsupported expect methods #11734

This is a powerful feature that allows tests to have conditional logic, whilst also ensuring that assertions in those conditions are always called (or the test will fail).