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 <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: llvm@lists.linux.dev
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240221034155.1500118-9-irogers@google.com