glsl/test: Don't run whitespace tests in parallel

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: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6301>