Fix hanging tests in integration suite

A recent change to nginx appears to be causing this test to fail to
cache the test file in the temp nginx instance we've spun up. This
causes the return of cp.cache_file to be False, which in and of itself
should be enough to fail the test. But since we're not asserting on the
cp.cache_file results, flow proceeds to the next block of code, in which
we open the "file" that we cached. Some genius thought it would be a
swell idea to make io.open() successfully open a filehandle when the
path passed to it is a boolean False. When you try to read from this
filehandle, an AMAZING thing happens... It just blocks. Forever. I know,
right? Pure genius.

This commit adds an assert so that the test fails gracefully and doesn't
try to read from a bogus filehandle that SHOULDN'T EVEN EXIST.

tests/integration/modules/test_cp.py

@@ -498,6 +498,7 @@ class CPModuleTest(ModuleCase):
             url = url_prefix + (code or 'actual_file')
             log.debug('attempting to cache %s', url)
             ret = self.run_function('cp.cache_file', [url])
+            self.assertTrue(ret)
             with salt.utils.fopen(ret) as fp_:
                 cached_contents = fp_.read()
                 self.assertEqual(cached_contents, file_contents)