Extend tests to detect shared libraries using system libraries

On some test machines, we saw test failures because openssl version mismatch:

```
Traceback (most recent call last):
  File "/srv/slapgrid/slappart14/srv/testnode/cqg/soft/18d28ecd49e5e0f732e2ce0c00c57519/parts/slapos.core-repository/slapos/testing/testcase.py", line 227, in installSoftwareUrlList
    checkSoftware(cls.slap, software_url)
  File "/srv/slapgrid/slappart14/srv/testnode/cqg/soft/18d28ecd49e5e0f732e2ce0c00c57519/parts/slapos.core-repository/slapos/testing/testcase.py", line 211, in checkSoftware
    raise RuntimeError('\n'.join(error_list))
RuntimeError: ./parts/proftpd/libexec/mod_auth_web.so:
./parts/proftpd/libexec/mod_auth_web.so: /usr/lib/x86_64-linux-gnu/libssl.so.1.1: version `OPENSSL_1_1_1' not found (required by /srv/slapgrid/slappart14/srv/testnode/cqg/inst/test0-0/tmp/shared/curl/724c785f86fff6993ff24ef745d4dbb9/lib/libcurl.so.4)
```

What we see in this traceback is that the version of openssl used was the one from system ( in `/usr/lib/` ), which should not happen because we are using setting rpath to slapos libraries. We had a test checking for missing rpath that could not resolved, but our test did not catch missing rpath that can be resolved on a system library, which is also wrong because this system library might be a different version like here or even be missing.

/reviewed-on nexedi/slapos.core!172