manifest/tests: get them passing under Windows

We also need to check more things in the manifest/project handlers,
and use platform_utils in a few places to address Windows behavior.

Drop Python 2.7 from Windows testing as it definitely doesn't work
and we won't be fixing it.

Change-Id: I83d00ee9f1612312bb3f7147cb9535fc61268245
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256113
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Jonathan Nieder <jrn@google.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>