CTestTestMemcheck: Fix matching of Guard Malloc messages

In commit 10bc50ea (Tests: ignore Guard Malloc messages in MemChecker
tests, 2013-05-13) we forgot to escape backslashes in the CMake language
to get them into the regex.  Add them now.
Merge topic 'cleanup-CTestTestMemcheck' into next

54a3e2a CTestTestMemcheck: Fix matching of Guard Malloc messages