Check for diff's availability

The tests suite calls `diff` in various places, which means we need to
check if it's installed.

testsuite/meson.build

@@ -2,6 +2,10 @@ gtk_libexecdir = join_paths(gtk_prefix, get_option('libexecdir'))
 installed_test_bindir = join_paths(gtk_libexecdir, 'installed-tests', 'gtk-4.0')
 installed_test_datadir = join_paths(gtk_datadir, 'installed-tests', 'gtk-4.0')
 
+# We call diff in various tests, so we need to check it's available,
+# otherwise we're going to have failures down the line
+diff = find_program('diff', required: true)
+
 subdir('gdk')
 subdir('gsk')
 subdir('gtk')