selftest/interpreter: Add tests for trusted_for(2) policies

selftest/interpreter: Add tests for trusted_for(2) policies

Test that checks performed by trusted_for(2) on file descriptors are
consistent with noexec mount points and file execute permissions,
according to the policy configured with the fs.trust_policy sysctl.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
Reviewed-by: Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220104155024.48023-5-mic@digikod.net