lib: tst_module: add library functions for kernel modules

There are new functions: tst_module_exists(), tst_module_load() and
tst_module_unload(). These functions help to load and unload kernel
modules in the tests.

Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
Reviewed-by: Caspar Zhang <caspar@casparzhang.com>
Reviewed-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>