libevent: don't treat test stats line as pass/fail in ptest

libevent: don't treat test stats line as pass/fail in ptest

Supplemental to commit fb17b46e2. The libevent "regress" test
outputs its own pass/fail results, e.g.
"2/300 TESTS FAILED. (31 skipped)", which will be miscounted
as an extra test fail in the ptest log. Fixed this to ignore
the libevent results line when counting actual pass/fail
results.

Also removed the for loop in run-ptest and targeted only the
libevent "regress" test, as the other tests being run were
related to performance and did not provide a relevant pass/fail
output.

(From OE-Core rev: 86b8a1d534bfcd70775c6e2b59eabe10de29f526)

Signed-off-by: Trevor Gamblin <trevor.gamblin@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>