PyTest.summaryf: better detection of summary line

If program output after pytest's summary line, our summary function
would be confused and detect 0 test run.
Make this matching more robust by expecting the actual status of tests
reported by pytest: (x)passed, (x)failed, skipped errors or "no test run"

From Kirill's suggestion on !17 (comment 162070)

/reviewed-by @kirr
/reviewed-on !17