test: make the MemoryHigh= limit a bit more generous with sanitizers
When we're running with sanitizers, sd-executor might pull in a
significant chunk of shared libraries on startup, that can cause a lot
of memory pressure and put us in the front when sd-oomd decides to go on
a killing spree. This is exacerbated further on Arch Linux when built
with gcc, as Arch ships unstripped gcc-libs so sd-executor pulls in over
30M of additional shared libs on startup: