Fix tests: use explicit syspathinsert where tests might hang (#7008)

Use `testdir.syspathinsert()` with multiprocessing tests:

- test_chained_exceptions_no_reprcrash
- test_exception_handling_no_traceback

This only works currently because `_importtestmodule` changes `sys.path`
as a side-effect.

It appears to be only required on Windows though - likely due to the
multiprocessing method used there.

testing/test_assertion.py

@@ -1264,6 +1264,7 @@ def test_exception_handling_no_traceback(testdir):
             multitask_job()
     """
     )
+    testdir.syspathinsert()
     result = testdir.runpytest(p1, "--tb=long")
     result.stdout.fnmatch_lines(
         [