Allow specify test names to testrunner

Allow specify test names to testrunner such as
testrunner.so tst-foo1.so tst-foo2.so
to run tst-foo1 and tst-foo2 only in the order
they are specified.
This does not change the default behavior that
cmdline=testrunner.so will run all tests under
/tests dir.
This change can help developer to run the only
test cases they need quickly to verify new feature
or find possible regressions.

Signed-off-by: Yang Bai <hamo.by@gmail.com>