fetch tests: re-arrange arguments for future readability

fetch tests: re-arrange arguments for future readability

Re-arrange the arguments to the test_configured_prune() function used
in this test to pass the arguments to --fetch last. A subsequent
change will test for more elaborate fetch arguments, including long
refspecs. It'll be more readable to be able to wrap those on a new
line of their own.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>