test/dm: don't use drop_caches
test/dm: don't use drop_caches

As we've moved the read tests to use O_DIRECT, dropping caches is no longer needed.

Signed-off-by: Jan Luebbe <3a82f1ac4ad59607a20f180f1c67ac8ba31ad0d5@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,jluebbe/rauc