Bail out of stack traversal when hitting unittest files (#421)

As at the status quo, stack traversal terminates when
hitting a file with 'pytest' or 'hammett' in the path,
but blithely carries on if a file has 'unittest' in
the path, despite that being the unittest framework.

The simple fix to that is to also bail out if
'unittest' is in the current frame's filename.

Author: CyberiaResurrection

mutmut/__main__.py

@@ -137,7 +137,8 @@ def record_trampoline_hit(name):
         f = inspect.currentframe()
         c = mutmut.config.max_stack_depth
         while c and f:
-            if 'pytest' in f.f_code.co_filename or 'hammett' in f.f_code.co_filename:
+            filename = f.f_code.co_filename
+            if 'pytest' in filename or 'hammett' in filename or 'unittest' in filename:
                 break
             f = f.f_back
             c -= 1