perf test: Make parallel testing the default

perf test: Make parallel testing the default

Now C tests can have the "exclusive" flag to run without other tests,
and shell tests can add "(exclusive)" to their description, run tests
in parallel by default. Tests which flake when run in parallel can be
marked exclusive to resolve the problem.

Non-scientifically, the reduction on `perf test` execution time is
from 8m35.890s to 3m55.115s on a Tigerlake laptop. So the tests
complete in less than half the time.

Tested-by: James Clark <james.clark@linaro.org>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Ilya Leoshkevich <iii@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Veronika Molnarova <vmolnaro@redhat.com>
Link: https://lore.kernel.org/r/20241025192109.132482-9-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>