tests: Update brightness atomically and wait longer
Sometimes the test_legacy_brightness_rounding seems to fail, in rare cases with the brightness file being empty. This is possible in busy CI environments due to the short time window (<= 200ms) and because the script first truncates the file.
First update the file atomically, to avoid weird errors. Second, delay the check a bit more in order to ensure we do not fail in busy CI environments.
Hopefully this avoids the flaky test seen in https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/jobs/1408861#L1171