[libunwind] [runtimes] Remove explicit -isysroot from the testing configurations on macOS (PR #66265)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> But even regardless of #67201, the intended way to use the libc++ test suite is to create a `.cfg.in` file that suits your needs if the general ones don't, not to add more configuration options to the general ones.

IMO, I think this is something I fundamentally disagree with.

Yes, the previous test setup with long intricate python classes was probably too messy/complex. But the other extreme - only ever having entirely static `.cfg` files isn't the best either (if that's required, I'm quite sure there'll be cases where people just skip caring about running the tests, rather than adding a config).

Some level of configurability for allowing injecting compiler flags in the existing test configs would be very welcome, IMO. It shouldn't evolve into the old mess of course, but just allowing adding something like `LIBCXX_TEST_CXXFLAGS`, for adding flags specifically to the test runs, would be quite useful for some test configs I would expect. (I currently don't have a need for the sysroot stuff touched upon here, but I get the demand for it.) E.g. kinda like building libc++ with `-std=c++23` but testing compiling the user code with `-std=c++17`. I guess that particular case is supported via some of the testsuite parameters, but users may want to build/test in other configurations as well - and as long as it's fundamentally the same as existing configs, having to create a new test config for it feels very much unnecessary.

https://github.com/llvm/llvm-project/pull/66265