test: make the MemoryHigh= limit a bit more generous with sanitizers

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:

~# lddtree build-san/systemd-executor
build-san/systemd-executor (interpreter => /lib64/ld-linux-x86-64.so.2)
    libasan.so.8 => /usr/lib/libasan.so.8
        libstdc++.so.6 => /usr/lib/libstdc++.so.6
        libm.so.6 => /usr/lib/libm.so.6
        libgcc_s.so.1 => /usr/lib/libgcc_s.so.1
    libsystemd-core-255.so => /root/systemd/build-san/src/core/libsystemd-core-255.so
        libaudit.so.1 => /usr/lib/libaudit.so.1
            libcap-ng.so.0 => /usr/lib/libcap-ng.so.0
...
    libseccomp.so.2 => /usr/lib/libseccomp.so.2
    libubsan.so.1 => /usr/lib/libubsan.so.1
    libc.so.6 => /usr/lib/libc.so.6

~# ls -Llh /usr/lib/libasan.so.8 /usr/lib/libstdc++.so.6 /usr/lib/libubsan.so.1
-rwxr-xr-x 1 root root 9.7M Feb  2 10:36 /usr/lib/libasan.so.8
-rwxr-xr-x 1 root root  21M Feb  2 10:36 /usr/lib/libstdc++.so.6
-rwxr-xr-x 1 root root 3.2M Feb  2 10:36 /usr/lib/libubsan.so.1

Sanitized libsystemd-core.so is also quite big:

~# ls -Llh /root/systemd/build-san/src/core/libsystemd-core-255.so /usr/lib/systemd/libsystemd-core-255.so
-rwxr-xr-x 1 root root  26M Feb  8 19:04 /root/systemd/build-san/src/core/libsystemd-core-255.so
-rwxr-xr-x 1 root root 5.9M Feb  7 12:03 /usr/lib/systemd/libsystemd-core-255.so