test: correctly configure the NUMA node memory pool

test: correctly configure the NUMA node memory pool

The currently hardcoded value works with the default configuration, but
breaks when QEMU_MEM != 512M (in sanitizer runs, for example).

```
# QEMU_MEM=1G make -C test/TEST-36-NUMAPOLICY/ run
make: Entering directory '/home/fsumsal/repos/@systemd/systemd/test/TEST-36-NUMAPOLICY'
TEST-36-NUMAPOLICY RUN: test NUMAPolicy= and NUMAMask= options
+ /bin/qemu-kvm -smp 8 -net none -m 1G -nographic -kernel /boot/vmlinuz-5.12.5-300.fc34.x86_64 -drive format=raw'
qemu-kvm: total memory for NUMA nodes (0x20000000) should equal RAM size (0x40000000)
E: QEMU failed with exit code 1
```