Fix 'cache flush_signed' test by disable offline compression in it (bug 1053178)

The test was failing for us locally because depending on the test order
some entries like consumer_info and site_config could have been written
to the offline cache.
Merge pull request #48 from diox/fix-cache-signed-flaky-test

Fix 'cache flush_signed' test by disable offline compression in it (bug 1053178)