I couldn't get all reftests to work reliably, so the tests failing
on either CI or on my machine are skipped for now.
Installed tests are disabled by default and can be enabled with "-Dinstalled_tests=true"
Under autotools the compiled schemas are in the build directory and with
meson they are in the root build dir. Avoid changing the autotools build for
now and add a special GTK_TEST_MESON env var which we can use to differentiate
the two.
See 1253e7bfcbc80f88 for a similar fix on master.
