MLBEDSW-10229: Fix unit test memory leak

 - Valgrind detects memory leaks in the unit tests from the function-static
   operator store. This commit allows the operator store to be cleaned up
   before a test exits.

Signed-off-by: Philip Hall <philip.hall@arm.com>
Change-Id: I2fdc0cf3bb1ddd390443a0c6588fb1a1d7b42702