res_sorcery_memory_cache: Execute stale unit test last.

In Jenkins there is currently a sporadic test failure of a
variable number of sorcery memory cache unit tests. I have not
been able to reproduce this on the build agents themselves or
on my development machine.

My working theory is that the stale unit test is causing a
sorcery instance to persist longer than expected, causing subsequent
tests to fail when setting up and initializing the next
sorcery instance.

To see if this is the case this change moves the stale unit test
to execute last so no subsequent unit tests can have issues
initializing their sorcery instance.

Change-Id: Ifd6550a949613be774b75fa5db12c02110f82c4a