perf tests: Add option to run tests in parallel

By default tests are forked, add an option (-p or --parallel) so that
the forked tests are all started in parallel and then their output
gathered serially. This is opt-in as running in parallel can cause
test flakes.

Rather than fork within the code, the start_command/finish_command
from libsubcmd are used. This changes how stderr and stdout are
handled. The child stderr and stdout are always read to avoid the
child blocking. If verbose is 1 (-v) then if the test fails the child
stdout and stderr are displayed. If the verbose is >1 (e.g. -vv) then
the stdout and stderr from the child are immediately displayed.

An unscientific test on my laptop shows the wall clock time for perf
test without parallel being 5 minutes 21 seconds and with parallel
(-p) being 1 minute 50 seconds.

Signed-off-by: Ian Rogers <[email protected]>
Cc: James Clark <[email protected]>
Cc: Justin Stitt <[email protected]>
Cc: Bill Wendling <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Yang Jihong <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Athira Jajeev <[email protected]>
Cc: [email protected]
Signed-off-by: Namhyung Kim <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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