Add support for custom unit test directory

/tmp/test being hard-coded into ep_testsuite.cc makes it
hard to have unit tests run on ramdisks. This change allows
the unit test suite to be pointed to a dir in ramdisk.

Change-Id: I2e428ed6fe1a38513db27a70aa7a1c36bbe90c6f
Reviewed-on: http://review.couchbase.org/38379
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>