tests: expose variables in meson.build

pygobject uses sources from gi's tests folder to build
libraries to test against, with autotools it worked by
obtaining gidatadir from pkg-config, but this is explicitly
prevented by meson's sandbox when gi is built as a subproject.
1 job for expose_meson_test_variables in 2 minutes and 30 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed build-job #25562

00:02:30