Clarify meaning of error vs failure

failure is assertion failure.
error is any other error.

Source: https://docs.python.org/3/library/unittest.html#organizing-test-code
-> "If the test fails, an exception will be raised ... identify the test
case as a failure. Any other exceptions will be treated as errors"

While searching ERP5 code for failure_count to clarify its meaning, I've
also discovered expected_failures and unexpected_successes (*) so they
are also added to the table.

(*) see e.g. https://lab.nexedi.com/nexedi/erp5/blob/1e31c091/product/ERP5Type/tests/ERP5TypeTestSuite.py#L6-8

nxdtest

@@ -267,10 +267,16 @@ class LocalTestResultLine:
         _ = v('error_count')
         if _ == '?':
             st = '?'
-        elif _ == 0:
-            st = 'ok'
+        elif _ != 0:
+            st = 'error'
         else:
+            _ = v('failure_count')
+            if _ == '?':
+                st = '?'
+            elif _ != 0:
                 st = 'fail'
+            else:
+                st = 'ok'
 
         print('%s\t%s\t%.3fs\t# %st %se %sf %ss' % (st, self.name, kw['duration'], v('test_count'), v('error_count'), v('failure_count'), v('skip_count')))
         # XXX + dump .json ?
@@ -290,17 +296,19 @@ class PyTest:
                 return default
             return int(m.group(1))
 
-        stat = {}
+        stat = {'test_count': 0}
         def stat_set(stat_key, from_name):
             v = get(from_name)
             if v is None:
                 return
             stat[stat_key] = v
+            stat['test_count'] += v
         stat_set('skip_count', 'skipped')
-        stat_set('error_count',   'failed')
-        stat_set('failure_count', 'xfailed')    # XXX ok?
-        npass = get('passed',  0)
-        stat['test_count'] = npass + stat.get('failure_count', 0) + stat.get('skip_count', 0) + stat.get('error_count', 0)
+        stat_set('failure_count', 'failed')
+        stat_set('expected_failures', 'xfailed')
+        stat_set('unexpected_successes', 'xpassed')
+        # don't set error_count - everything goes to failure_count
+        stat['test_count'] += get('passed',  0)
         return stat