test remote configuration against temporary repositories

Cover repository detection, fetch URL creation and changes, implicit push
preservation, idempotent adds, removal and collapsing, unknown remotes and
URLs that would be dangerous if git-fleet built shell command lines. Every
test runs in a temporary repository.

test/dune

@@ -1,3 +1,3 @@
 (test
  (name test_git_fleet)
- (libraries git_fleet alcotest))
+ (libraries git_fleet alcotest unix))