Reset test duration at start of test

After comments by @Otto-AA, who pointed out that although my
changes are good _now_, they would be less likely to break in
future if we explicitly reset the accumulated test duration,
so this commit does just that.

Author: CyberiaResurrection

mutmut/__main__.py

@@ -417,6 +417,10 @@ def execute_pytest(self, params: list[str], **kwargs):
 
     def run_stats(self, *, tests):
         class StatsCollector:
+            # noinspection PyMethodMayBeStatic
+            def pytest_runtest_logstart(self, nodeid, location):
+                mutmut.duration_by_test[nodeid] = 0
+
             # noinspection PyMethodMayBeStatic
             def pytest_runtest_teardown(self, item, nextitem):
                 unused(nextitem)