testing/tests/download: test git submodules

Add two submodules as static repos, add a branch to the main static repo
and check on the git refs test case the download of a git package:
 - repo with submodule but without support in the package;
 - repo with recursive submodules with support in the package.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Cc: Arnout Vandecappelle <arnout@mind.be>
[Arnout: remove handling of inconsistent tarball hashes - that's an
 actual bug that should be fixed]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

support/testing/tests/download/git-remote/refs-sub1.git/objects/cb/545facf77bbc5f24f95b6d503c338d10b7b717

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