glsl/test: Don't run whitespace tests in parallel
For some unfathomable reason, three out of these four tests often time out when running within CI. On the assumption that there is some parallelisation badness happening rather than the non-UNIX tests entering infinite loops, try just marking them as serial-only. This should have a negligible impact on runtime since they are quick to execute. Signed-off-by:Daniel Stone <daniels@collabora.com> Reviewed-by:
Eric Engestrom <eric@engestrom.ch> Part-of: <mesa/mesa!6301>