tests/shell: unload modules between tests

tests/shell: unload modules between tests

This patch adjusts the main test script so it unload all nftables
kernel modules between tests.

This way we achieve two interesting things:
 * avoid false errors in some testcases due to module loading order
 * test the module loading/unloading path itself

The false positives is for example, listing ruleset per families, which depends
on the loading order of nf_tables_xx modules.

We can later add more modules to unload incrementally (for
example nf_tables_switchdev).

This patch assumes we are working with a kernel which is compiled with
nf_tables =m, the case using =y is not supported and can still produce false
positives in some testcases due to module ordering.

Reported-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>