Attempt to fix flaky test on Python 2

[nicoddemus]

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.

Example of a recent failure: https://ci.appveyor.com/project/pytestbot/pytest/build/1.0.4327/job/yfunwupo1bo3rvdc

[coveralls]

Coverage increased (+0.05%) to 92.821% when pulling fa76a5c on nicoddemus:fix-flaky-xdist-test into 27651f4 on pytest-dev:master.