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.
    
    (cherry picked from commit f232c83c7257e08ccbeafc994c90e38515d04c6e)