A full build of protobuf depends on gmock even though it is not part of
a standrad checkout. This CL explicitly disable the build of the
protobuf tests to get rid of this dependency.

If somebody want to build the protobuf tests then they have to download
gmock to the protobuf directory and specify -Dprotobuf_BUILD_TESTS=ON
to the cmake command line.

Fixes https://github.com/grpc/grpc/issues/7233