test_file_download.py: Don't make > 40 client calls in a test

[jku]

test_multiple_changes_to_target starts the client 43 times, this is excessive and not useful.

• Keep making multiple versions of the repository metadata
• Only use the client every 5 targets versions

Also avoid unneeded refresh() calls in all target download tests.

All in all this means ~30 fewer client startups.