cmake: set pre and post memcheck commands.

We need to clear out and re-create the tmp directory when running
valgrind just like for the normal test run.

cmake/CTestCustom.cmake

@@ -1,2 +1,4 @@
 set(CTEST_CUSTOM_PRE_TEST "sh -c \"rm -rf ../test/tmp && mkdir ../test/tmp\"")
 set(CTEST_CUSTOM_POST_TEST ${CTEST_CUSTOM_PRE_TEST})
+set(CTEST_CUSTOM_PRE_MEMCHECK ${CTEST_CUSTOM_PRE_TEST})
+set(CTEST_CUSTOM_POST_MEMCHECK ${CTEST_CUSTOM_PRE_TEST})