Add xvfb to dependencies in README.md

[Tbusk]

Closes #501

This PR adds xvfb to the list of dependencies in the readme.

[stsdc]

But this is a test dependency right?
Just wondering if there should a separate list for test deps...

[Tbusk]

Perhaps, although, the particular dependency seems to force its usage upstream to this project due to it being set as default.

Meaning if any tests are ran in this repo, it forces it to be wrapped by the xvfb runner.

Should it get its own section @stsdc ?

[stsdc]

I mean we can add this, but i think the real problem is that we can't disable live-chart tests.

I think it would be nice to find some solution to that.

But if there is no solution, I approve anyway.

[Tbusk]

Are you referring to in the CI, specifically the commented out test lines here @stsdc?

https://github.com/elementary/monitor/blob/main/.github/workflows/ci.yml

If so, yeah, that is easy to get around.

Although I did run into an issue where all my tests were being wrapped in xvfb due to the depenency of livecharts setting it as default and it propagating, causing my tests to fail. At least now, the existing test isn't showing that.

Then all you need to do is (at least for now) modify it to run the specific test (and perhaps a test suite later on) https://mesonbuild.com/Unit-tests.html

 meson test -C build monitor-statusbar-test

And the test would result in this (once the typo/gtk init signature change is fixed in the test):

[stsdc]

yeah. Then, let's create a separate workflow for test purposes and isolate Monitor's tests there.