[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.