tests: Add ability to test a branch other than master

This commit makes it so that a unit test can create the test app and
runtime using a branch other than master, and changes test-run.sh to use
the branch "stable". This will allow the run command to be tested better
in the following commit.

Closes: #2788
Approved by: matthiasclasen
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f