add memory tox target

this is for future compat w/ the nox change

Change-Id: Ide0c24457ca6c1b305b7cb4a7d946ed732fb2065

Author: zzzeek

tox.ini

@@ -26,6 +26,8 @@ setenv=
     {valkey_sentinel}: PIFPAF=pifpaf --env-prefix DOGPILE run valkey --port {env:TOX_DOGPILE_PORT:11234} --sentinel --sentinel-port {env:TOX_DOGPILE_SENTINEL_PORT:11235} --
     {valkey_sentinel}: RUNTESTS=tests/cache/test_valkey_sentinel_backend.py
 
+    {memory}: RUNTESTS=tests/cache/test_memory_backend.py
+
     {dbm}: RUNTESTS=tests/cache/test_dbm_backend.py
 
     {cov}: COVERAGE={[testenv]cov_args}