[CI] Reduce file count in gfal2 system tests

Description

This MR:

• reduce the file count for the gfal tests to 1k each (so 2k in total for both gfal2-plugin-xrootd and gfal2-plugin-http
• reduce the number of processes in both test_client_gfal2.sh and test_client.sh from 100 to 20 (NB_PROCS)
  • 100 is way too much and just causes unnecessary contention
• removes a few redundant sleeps

Checklist

• Documentation reflects the changes made.
• Merge Request title is clear, concise, and suitable as a changelog entry. See this link

References

Closes #1237 (closed)