Rename .upstream-tests

[brycelelbach]

Especially after the refactoring away from submodules, I find the name of this directory very confusing; the upstream tests (e.g. raw libc++ tests) live in libcxx/test, not .upstream-tests.

[griwes]

I'm planning to do away with the subdirectory entirely and just promote the symlinks to be the actual directories inside .upstream-tests.

[griwes]

Also if I remember my reasoning back when it was added, there was some kind of an expectation that we'd have some CUDA-specific tests outside of the libc++ test framework, so the name isn't "tests of the upstream", it's "tests that come from the upstream". But I agree that in hindsight, it's not the best name imaginable ;)

[jrhemstad]

#1075