tests: move away from use of hardcoded `master`

[skshetry]

When I do git init, it prints the following warning:

$ git init
hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change.

So, I don't think we should depend on the hardcoded default branch in our tests in the future. To do so, we should introduce scm.default_branch property and use that in the tests instead.

Originally posted by @skshetry in #6857 (comment)

[skshetry]

Closing as not planned.