tests: Provide G_TEST_SRCDIR, G_TEST_BUILDDIR to lint scripts
These scripts use $(readlink -f)
to guess their own path if necessary,
but macOS readlink doesn't support the -f option, and POSIX doesn't
guarantee that readlink even exists.
Fixes: d7601f7e "Incorporate some lint checks into meson test
"
Unfortunately macOS CI was down when I sent in !3898 (merged), so this wasn't detected at the time.
Edited by Simon McVittie