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

This reverts commit c60cea0daa.

Didn't have the intended effect, and slowed down the meson test run.

Reviewed-by: Dylan Baker <dylan.c.baker@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9528>

src/compiler/glsl/glcpp/meson.build

@@ -83,13 +83,6 @@ glcpp = executable(
 if with_any_opengl and with_tests and host_machine.system() != 'windows'
   modes = ['unix', 'windows', 'oldmac', 'bizarro']
 
-  # 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.
   foreach m : modes
     test(
       'glcpp test (@0@)'.format(m),
@@ -101,7 +94,6 @@ if with_any_opengl and with_tests and host_machine.system() != 'windows'
       ],
       suite : ['compiler', 'glcpp'],
       timeout: 60,
-      is_parallel: false,
     )
   endforeach