[libc] Skip tests that depend on undefined targets

When we search through the dependencies for a test we somtimes query
`get_target_property` on a target that has not been defined. This is the
case for targets that have minimal entrypoints defined such as the WIP
GPU target. This patch changes all tests whose dependencies are
undefined to be skipped entirely.

Reviewed By: sivachandra, lntue, michaelrj

Differential Revision: https://reviews.llvm.org/D139216