Use monkeypatch to set strict_exception_groups

The pytest tester does not run tests in a subinterpreter or subprocess,
so changing trio.run in a testdir test will also change the global
trio.run. Monkeypatching resets trio.run after the test.

Author: VincentVanlaer

pytest_trio/_tests/test_basic.py

@@ -1,4 +1,6 @@
+import functools
 import pytest
+import trio
 
 
 def test_async_test_is_executed(testdir):
@@ -73,15 +75,17 @@ def test_invalid():
     result.assert_outcomes(errors=1)
 
 
-def test_skip_and_xfail(testdir):
+def test_skip_and_xfail(testdir, monkeypatch):
+    monkeypatch.setattr(
+        trio, "run", functools.partial(trio.run, strict_exception_groups=True)
+    )
+
     testdir.makepyfile(
         """
         import functools
         import pytest
         import trio
 
-        trio.run = functools.partial(trio.run, strict_exception_groups=True)
-
         @pytest.mark.trio
         async def test_xfail():
             pytest.xfail()