summaryf needs to always return the count of failures, errors, skips
for two reasons:

 - nxdtest sets error_count += 1 when the test program's returncode is
not zero, so if we don't set error_count explicitly, the number of
errors will be 1 when there are only failures
 - without error_count / failure_count, _test_result_summary does not
display the global test result as pass or fail but as ?

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