test: skip a harmless ldd error when installing test libs

Skip a harmless error when running the tests on a system with a significantly
older systemd version (ldd tries to resolve the unprefixed RPATH for libsystemd.so.0,
which is in this case older than the already installed libsystemd.so.0 in $initdir).
The issue is triggered by installing test dependencies in install_missing_libraries().

Spotted on CentOS 8.

```
$ ldd /var/tmp/systemd-test.nZO11F/root/lib/systemd/tests/test-sd-device-thread
/var/tmp/systemd-test.nZO11F/root/lib/systemd/tests/test-sd-device-thread: /lib64/libsystemd.so.0: version `LIBSYSTEMD_240' not found (required by /var/tmp/systemd-test.nZO11F/root/lib/systemd/tests/test-sd-device-thread)
        linux-vdso64.so.1 (0x00007fffb79d0000)
        libclang_rt.asan-powerpc64le.so => /usr/lib64/clang/11.0.0/lib/linux/libclang_rt.asan-powerpc64le.so (0x00007fffb6ef0000)
        libsystemd.so.0 => /lib64/libsystemd.so.0 (0x00007fffb6d20000)
        libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fffb6cd0000)
        libc.so.6 => /lib64/libc.so.6 (0x00007fffb6ab0000)

$ LD_LIBRARY_PATH=/var/tmp/systemd-test.nZO11F/root/lib64/ ldd /var/tmp/systemd-test.nZO11F/root/lib/systemd/tests/test-sd-device-thread
        linux-vdso64.so.1 (0x00007fffaba80000)
        libclang_rt.asan-powerpc64le.so => /usr/lib64/clang/11.0.0/lib/linux/libclang_rt.asan-powerpc64le.so (0x00007fffaafa0000)
        libsystemd.so.0 => /var/tmp/systemd-test.nZO11F/root/lib64/libsystemd.so.0 (0x00007fffaa5f0000)
        libpthread.so.0 => /var/tmp/systemd-test.nZO11F/root/lib64/libpthread.so.0 (0x00007fffaa5a0000)
        libc.so.6 => /var/tmp/systemd-test.nZO11F/root/lib64/libc.so.6 (0x00007fffaa380000)

```