LU-13432 tests: test-framework to preserver MODOPTS_LIBCFS

LU-13432 tests: test-framework to preserver MODOPTS_LIBCFS

do not reset MODOPTS_LIBCFS at module loading so that
if some one passes extra option to a test that's preserved
over a whole run rather than reset that at the first module
reload.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I235d09dc7ed4084b95e7f85528b952b75b16744d
Reviewed-on: https://review.whamcloud.com/38173
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>