Tests: fix build of dummy memtester on AIX

Both <string> and <cmSystemTools.h> include <stdio.h> one or the other way. It
looks like the CMake header adds a define that will cause another part of the
system header to be used, resulting in conflicting definitions.
Merge topic 'Memchecker-AIX-fix' into next

1a2a6ba Tests: fix build of dummy memtester on AIX