Switch from running tests sequentially to running in parallel by
default. Change the opt-in '-p' or '--parallel' flag to '-S' or
'--sequential'.

On an 8 core tigerlake an address sanitizer run time changes from:

  326.54user 622.73system 6:59.91elapsed 226%CPU

to:

  973.02user 583.98system 3:01.17elapsed 859%CPU

So over twice as fast, saving 4 minutes.
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240301174711.2646944-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>