UnitTestFrameworkPkg/PersistenceLib: Correct the allocated size.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2608



According to logic and the practice, it is need to allocate ascii length
by 2 for unicode string.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Guomin Jiang <guomin.jiang@intel.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>