Provide default cache config for unittesting APC and File drivers

Since 1fdb7e3c10 the module does not provide any default
config, so cache driver tests were mostly being skipped.
Provide default configs within the unit tests so that these
tests can run.

Previously, the APC test was not checking for existence of
the config causing travis build failures on HHVM (where APC
is installed by default) but passes on other PHP versions
because APC is not present.