Attempt to fix flaky test on Python 2

This test sometimes fails on AppVeyor with:

```
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
.0 = <listiterator object at 0x0549DC50>
    leaked_types = sum(1 for _ in gc.garbage
>                   if 'PseudoFixtureDef' in str(_))
E   UnicodeEncodeError: 'ascii' codec can't encode character u'\u263a' in position 23: ordinal not in range(128)
test_request_garbage.py:19: UnicodeEncodeError
====================== 1 passed, 1 error in 1.39 seconds ======================
```

Use our internal "safe_repr" function to handle Encode errors: who knows
what objects are in garbage after all.

testing/python/fixture.py

@@ -523,6 +523,7 @@ def test_request_garbage(self, testdir):
         testdir.makepyfile("""
             import sys
             import pytest
+            from _pytest.compat import safe_str
             import gc
 
             @pytest.fixture(autouse=True)
@@ -539,7 +540,7 @@ def something(request):
 
                     gc.collect()
                     leaked_types = sum(1 for _ in gc.garbage
-                                    if 'PseudoFixtureDef' in str(_))
+                                       if 'PseudoFixtureDef' in safe_str(_))
 
                     gc.garbage[:] = []