[CI] Reduce file count in gfal2 system tests

In test_client_gfal2.sh we do the complete archive, retrieve, evict, delete workflow twice. Once with gfal2-plugin-xrootd and once with gfal2-plugin-http. This is done with 5k files for both (so 10k in total).

There seems to be some sort of inefficiency in the test making this rather slow for 5k. Since these tests are primarily functional, we should decrease this to 1k files.

This will speed up this particular test by quite a bit and also reduce its chance of failure due to the double free (see #776 (closed))