Write a meta-test suite of test.data.table() + test() #6109

          this is essentially a regression test on `test.data.table()` and/or `test()`, which we don't have in the suite yet (the whole file is marked `# nocov`). It doesn't quite match the normal way of doing tests for us, and I'm not an immediate fan of nesting `test.data.table()` within `test.data.table()`... let's just revert the new tests in this PR.

I'll file a follow-up that we should really write some meta-tests of test.data.table() itself, at some point.

Originally posted by @MichaelChirico in #6103 (comment)

Then we can remove # nocov on that file. Having regression tests on the behavior of test()/test.data.table() themselves is good practice to ensure nothing breaks when refactoring their implementations. The existing test suites of 10K+ tests do serve as a good implicit regression test of the functionality, so this is not top priority at the moment.