test_runner: detect only tests when isolation is off

[cjihrig]

test_runner: apply filtering when tests begin

This commit updates the way filtering is applied to tests and
suites. After this change, filters are applied just before the
test/suite is started. The results are the same, but this allows
us to eventually move away from the --test-only flag except
when process level isolation is used.

test_runner: detect only tests when isolation is off

This commit updates the way the test runner processes 'only'
tests when process-based test isolation is disabled. The
--test-only flag is no longer necessary in this scenario. The
test runner will automatically detect 'only' tests and apply the
appropriate filtering.

This is not a breaking change because disabling test isolation is currently an experimental feature.