Run test_request_garbage in a subprocess

This test fails on occasion, and one suspicion is that some state from
the outer pytest might be leaking into the test.

testing/python/fixture.py

@@ -542,7 +542,7 @@ def test_func():
                 pass
         """
         )
-        result = testdir.runpytest()
+        result = testdir.runpytest_subprocess()
         result.stdout.fnmatch_lines("* 1 passed in *")
 
     def test_getfixturevalue_recursive(self, testdir):