Adjust duration_by_test definition to reflect new use

As at the status quo, whichever call item ran last (usually
'teardown') had its time overwrite the test in question's entry
in duration_by_test.

The simplest way I could think of to avoid that was to add
each call item's duration to the total in duration_by_test.
Changing duration_by_test to a defaultdict simplified the
bookkeeping.

Author: CyberiaResurrection

mutmut/__init__.py

@@ -3,7 +3,7 @@
 __version__ = '3.3.1'
 
 
-duration_by_test = {}
+duration_by_test = defaultdict(float)
 stats_time = None
 config = None
 

mutmut/__main__.py

@@ -426,7 +426,7 @@ def pytest_runtest_teardown(self, item, nextitem):
 
             # noinspection PyMethodMayBeStatic
             def pytest_runtest_makereport(self, item, call):
-                mutmut.duration_by_test[item.nodeid] = call.duration
+                mutmut.duration_by_test[item.nodeid] += call.duration
 
         stats_collector = StatsCollector()