apparmor: test: make static symbols visible during kunit testing

Use macros, VISIBLE_IF_KUNIT and EXPORT_SYMBOL_IF_KUNIT, to allow
static symbols to be conditionally set to be visible during
apparmor_policy_unpack_test, which removes the need to include the testing
file in the implementation file.

Change the namespace of the symbols that are now conditionally visible (by
adding the prefix aa_) to avoid confusion with symbols of the same name.

Allow the test to be built as a module and namespace the module name from
policy_unpack_test to apparmor_policy_unpack_test to improve clarity of
the module name.

Provide an example of how static symbols can be dealt with in testing.

Signed-off-by: Rae Moar <rmoar@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>