Fix #315 run brew test in CI

[peso]

This Pull Request fixes/closes #315 .

It changes the following:

• CI script has a new job that build gitui from source at head, using brew, and runs brew test

I followed the checklist:

• [no] I added unittests
• [no] I ran make check without errors
• [no] I tested the overall application
• [no] I added an appropriate item to the changelog

..but, I did make a custom tap of brew, introduced an error in the source code on my fork and let my custom brew discover it.

The thing is, there is no change to the source code of gitui, only to the CI script. The change looks at gitui main branch head, which I cannot touch - so I had to hack around to fake a test. I have demonstrated that it works on my fork, but not that it works on gitui master branch.

[extrawurst]

@peso thanks for looking into this. any idea why this checks out 0.7.0? see https://github.com/gitui-org/gitui/actions/runs/14307675619/job/40103031807?pr=2596#step:8:42

[peso]

The gitui 0.7.0 puzzled me as well, but it is simply test data for the brew ui test.
The brew test clones the repository at that commit, opens it with gitui, and expects to see the author of the commit on the screen.
It could have been any commit in any repository, as long as the test was updated to reflect what was in the commit.

[extrawurst]

@peso thank you!