ci: Run a maximum of 4 tests simultaneously
This is an attempt to deal with the random test failures that sometimes appear, e.g. https://gitlab.gnome.org/GNOME/tracker/-/jobs/432429 The CI runners on gitlab.gnome.org report 32 CPU cores, so Meson tries to run 32 tests at a time. I guess because our tests are mostly IO bound this actually makes things slower and causes random failures.
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