file: run test suite when building natively

As we apply the same patches to native and target builds of file, we can verify
that the patches are not breaking by executing the test suite during the build
of file-native.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>