[gdb/testsuite] Fix DUPLICATEs in s390-multiarch.exp

On s390x-linux, I run into:
...
DUPLICATE: gdb.arch/s390-multiarch.exp: Linux v2
DUPLICATE: gdb.arch/s390-multiarch.exp: Linux v2
DUPLICATE: gdb.arch/s390-multiarch.exp: Linux v2
...

Fix this by using with_test_prefix.

Tested on s390x-linux.
1 file changed