Add regression tests for keys on nested associations

This adds the regressions tests from PR #15893 to master. It's checking that both strings and symbols are accepted as keys for nested associations.
@sgrif not sure if this gives more background or clarify into the test - but it came from this issue (#15671).

I'm definitely not against improving the test further, btw. Just want you to see where I was coming from in making the tests.

What do you think a better test name would be? :smile: