validate: launcher: add --fair-sched=try to valgrind args

Enables fairer scheduling of threads in valgrind, possibly
at cost of performance though.

Hopefully helps with valgrind core/base jobs deadlocking or
timing out when runners are not under load.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/7753>

subprojects/gst-devtools/validate/launcher/baseclasses.py

@@ -638,6 +638,7 @@ class Test(Loggable):
 
         for o, v in [('trace-children', 'yes'),
                      ('tool', 'memcheck'),
+                     ('fair-sched', 'try'),
                      ('leak-check', 'full'),
                      ('leak-resolution', 'high'),
                      # TODO: errors-for-leak-kinds should be set to all instead of definite