drm/tests: Change "igt_" prefix to "drm_test_"

With the introduction of KUnit, IGT is no longer the only option to run
the DRM unit tests, as the tests can be run through kunit-tool or on
real hardware with CONFIG_KUNIT.

Therefore, remove the "igt_" prefix from the tests and replace it with
the "drm_test_" prefix, making the tests' names independent from the tool
used.

Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Acked-by: David Gow <davidgow@google.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20220911191756.203118-2-mairacanal@riseup.net