asn1_test: Enable the testcase only when GRUB_LONG_MAX is larger than GRUB_INT_MAX

asn1_test: Enable the testcase only when GRUB_LONG_MAX is larger than GRUB_INT_MAX

There is a testcase to test the values larger than "int" but smaller
than "long". However, for some architectures, "long" and "int" are the
same and the compiler may issue a warning like this:

grub-core/tests/asn1/tests/Test_overflow.c:48:50: error: left shift of negative value [-Werror=shift-negative-value]
       unsigned long num = ((long) GRUB_UINT_MAX) << 2;
                                                  ^~

To avoid unnecessary error the testcase is enabled only when
GRUB_LONG_MAX is larger than GRUB_INT_MAX.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Gary Lin <glin@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Stefan Berger <stefanb@linux.ibm.com>