perf test: Reintroduce -p/--parallel and make -S/--sequential the default

We can't default to doing parallel tests as there are tests that compete
for the same resources and thus clash, for instance tests that put in
place 'perf probe' probes, that clean the probes without regard to other
tests needs, ARM64 coresight tests, Intel PT ones, etc.

So reintroduce --p/--parallel and make -S/--sequential the default.

We need to come up with infrastructure that state which tests can't run
in parallel because they need exclusive access to some resource,
something as simple as "probes" that would then avoid 'perf probe' tests
from running while other such test is running, or make the tests more
resilient, till then we can't use parallel mode as default.

While at it, document all these options in the 'perf test' man page.

Reported-by: Adrian Hunter <[email protected]>
Reported-by: Arnaldo Carvalho de Melo <[email protected]>
Reported-by: James Clark <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/lkml/Ziwm18BqIn_vc1vn@x1
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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