t/t3905: use the name 'actual' for test output, swap arguments to test_cmp

t/t3905: use the name 'actual' for test output, swap arguments to test_cmp

It is common practice in the git test suite to use the file names 'actual'
and 'expect' to hold the actual and expected output of commands.  So change
the name 'output' to 'actual'.

Additionally, swap the order of arguments to test_cmp when comparing
expected output and actual output so that if diff output is produced, it
describes how the actual output differs from what was expected rather than
the other way around.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>