testsuite: Fix build warning on kernel 6.15

testsuite: Fix build warning on kernel 6.15

The module description is now required, otherwise a warning is emitted
by modpost:

	WARNING: modpost: missing MODULE_DESCRIPTION() in mod-loop-k.o

Fix all of them with:

	sed -i '/MODULE_LICENSE("LGPL");/a MODULE_DESCRIPTION("dummy test module");' testsuite/module-playground/mod-*.c
	sed -i '/MODULE_LICENSE("GPL");/a MODULE_DESCRIPTION("dummy test module");' testsuite/module-playground/mod-simple.c

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>