Remove failing SafeFixture tests

Remove failing SafeFixture tests

The fixtures 1.3 release attempted to fix the fixtures resource
leak issue, but failed to do so completely. Our own SafeFixture
is still needed: The 1.3 release broke our SafeFixture tests,
but not the usage of SafeFixture itself. This patch removes
those failing tests for now to unbreak the gate. Jakub reported
a bug on fixtures 1.3:
https://bugs.launchpad.net/python-fixtures/+bug/1469759

We will continue to use SafeFixture until that bug is fixed
in fixtures, at which point we will be able to require
fixtures > 1.3.

Change-Id: I59457c3bb198ff86d5ad55a1e623d008f0034b8f
Closes-Bug: #1469734