[check-licenses] Updates to the 'fix' subcommand tests

Added unit tests for output verification and failure cases in
fix_test.go.

Fixed Go flag resetting behavior in tests by setting fuchsiaDir AFTER
parsing.

Change-Id: I7f81f585dad94dd2e1d7a4e4b8aa27a77790dfda
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1602768
Reviewed-by: Jiaming Li <lijiaming@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>