Restore missing runtimes-test-depends target that causes build failures when...

Restore missing runtimes-test-depends target that causes build failures when LLVM_INCLUDE_TESTS is ON

7cc8377f removed the 'runtimes-test-depends' target in runtimes builds that
is assumed to exist when using a bootstrapped runtime build.

For a full analysis, see:
https://discourse.llvm.org/t/looking-for-guidance-on-broken-downstream-bootstrapped-runtimes-builds/62934

Differential Revision: https://reviews.llvm.org/D127325