tests: link tests using fabs against libm explicitly

tests: link tests using fabs against libm explicitly

Some compiler wrappers like honggfuzz pass -fno-builtin explicitly
and because of that the tests where fabs is used fail to compile
with something like
```
FAILED: test-bus-marshal
...
/usr/bin/ld: test-bus-marshal.p/src_libsystemd_sd-bus_test-bus-marshal.c.o: undefined reference to symbol 'fabs@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib64/libm.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```

Fun fact: it took honggfuzz less than a minute to discover
https://github.com/advisories/GHSA-gmc7-pqv9-966m used by
systemd to compress/descompress some stuff.