[libFuzzer] Use custom target instead of list of binaries for tests.

Update cmake to use a custom target TestBinaries instead of a list of targets.
This simplifies cmake, and fix some errors. This way, we don't have to propagate
the values into parents directories. We only need to use add_dependencies.

Differential Revision: https://reviews.llvm.org/D29593

llvm-svn: 294389