[gn] Add dummy build file for SPIRVTests

sync_source_lists_from_cmake.py checks that every unittest in CMake
also exists in the GN build. 7b08b436 added SPRIVTests, but the GN
build doesn't include the SPIRV target. So add a dummy target for this
to placate the check.

See also VETests in the gn build, which has the same setup.