build: Use add_test_setup() to allow tests to be run under valgrind
This means we can specify the standard options for testing GLib under
valgrind consistently, so that developers can use meson test --setup=valgrind
to run them.
Port the existing valgrind CI to use them (this will not change its functional behaviour).
Suggested by Marco Trevisan at !2717 (comment 1478891).
Signed-off-by: Philip Withnall pwithnall@endlessos.org