Merge all the llvm-exegesis unit tests into a single binary

These tests access private symbols in the backends, so they cannot link
against libLLVM.so and must be statically linked.  Linking these tests
can be slow and with debug builds the resulting binaries use a lot of
disk space.

By merging them into a single test binary means we now only need to
statically link 1 test instead of 6, which helps reduce the build
times and saves disk space.

Reviewed By: courbet

Differential Revision: https://reviews.llvm.org/D106464