feat: add WN file access test

File access is one of the key features in site's performance, so it's good to have a SAM test that monitors this aspect. Here we refactor an outdated test that checks availability of the local storage by copying test file from WN to the "nearby" storage and back.

To determine nearby storage we use the following mechanism: first site name is determined either from SITE_NAME environment variable or from WN's FQDN. When the site name is known we query GOCDB for a special extension to get test directory. If this does not work we use static configuration.

If nothing works, and we are unable to determine nearby storage, CERN's storage is used for the test, since the production jobs should be able to access CERN storage anyway (for example, to upload the logs).

I use my own user space directory for the tests for now, this probably should be changed if the changes are to be put in production.

For the following sites I was not able to find writable path (with user proxy extensions): NCBJ NIPNE CBPF This can probably be fixed by adding production extensions to the proxy or by ticketing the site.

For the following sites it was impossible to determine nearby storage, so CERN's EOS SE was used: CSCS Glasgow IN2P3 LAL LPNHE This can be fixed by ticketing the site and asking them to publish SITE_NAME variable (if these tests is to proceed). More complicated case is LAL and LPNHE -- these sites have the same name in GOCDB (GRIF) and the same domain names, so it is difficult to distinguish them..