mtd: make mtd_test.c a separate module

This file gets linked into nine different modules, which causes a warning:

scripts/Makefile.build:236: drivers/mtd/tests/Makefile: mtd_test.o is added to multiple modules: mtd_nandbiterrs mtd_oobtest mtd_pagetest mtd_readtest mtd_speedtest mtd_stresstest mtd_subpagetest mtd_torturetest

Make it a separate module instead.

Fixes: a995c792 ("mtd: tests: rename sources in order to link a helper object")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240529095049.1915393-1-arnd@kernel.org