Improve cache director-based tests and self-tests names and cleanup

[haoming29]

Currently, we put all file transfer tests for caches under /pelican/monitoring prefix, with tests from the director named in RFC3399 date format, e.g. 2024-05-14T15:39:38Z.txt and self-test named by an additional prefix: self-test-2024-05-14T15:39:38Z.txt. The director-test name is difficult to read/understand and it should also contain a prefix.

Also, to separate out the test folders, we should have director tests falls under /pelican/monitoring/directorTest while self-test falls under /pelican/monitoring/selfTest.

In #1262, the root cause is that we changed Cache.DataLocation to Cache.RootLocation that breaks backward compatibility. When the cache create the self-test folder, it didn't check if Cache.DataLocation is non-empty (in this case it is), and it creates a folder /pelican/monitoring, which is not a valid xrootd namespace folder. When the cache requests a self-test file, it can't find it locally, and thus contact the director for such a file, i.e. /pelican/monitoring/self-test-xxxx.txt and the director, thought the cache was asking for the director-based test file, gave the cache a file for director-based test, the content of which mismatches with the one for self-test.

With the above enhancement in place, we should be able to filter out irrelevant requests to the director, i.e. requests to /pelican/monitoring/xxx.txt and requests to /pelican/monitoring/directorTest/xxx.txt where xxx does not contain a valid prefix and format, so that the director can reject redirecting the request, reducing false positives.