[llvm-cov][test] Add test coverage for "gcov" implying "llvm-cov gcov" compatibility.

Much like other LLVM binary utilities, `llvm-cov` has a symlink compatibility feature where it runs in `gcov` compatibility mode if the binary name ends in `gcov`. This is identical to invoking `llvm-cov gcov ...`.

Differential Revision: https://reviews.llvm.org/D102299