PyTest.summaryf: always report count of failures and errors

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 ?
1 job for fix/test-failures in 0 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  External
passed nxdtest.UnitTest-Master.Python3-WIP

00:01:48