Add a way to skip the Go bindings tests even when Go is configured in

CMake.

The Go bindings tests in an unoptimized build take over 30 seconds for
me, making it the slowest test in 'check-llvm' by a factor of two.

I've only rigged this up fully to the CMake build. If someone is
interested in rigging it up to the autoconf build, they're welcome to do
so.

llvm-svn: 247243