[runtimes] Ensure required deps for tests targets are actually built

When building compiler-rt via runtimes, many tests fail because tools like
FileCheck and count aren't built yet. This is because the RUNTIME_TEST_DEPENDENCIES
haven't been added to any of the compiler-rt targets. The fix is to explicitly
add any runtimes as test_targets.

Differential Revision: https://reviews.llvm.org/D109625