[gdb/testsuite] Fix DUPLICATEs in gdb.dwarf2/implptr-64bit.exp

When running test-case gdb.dwarf2/implptr-64bit.exp with target board
unix/-m32, I noticed:
...
DUPLICATE: gdb.dwarf2/implptr-64bit.exp: failed to prepare
...

Fix this by using with_test_prefix.

Tested on x86_64-linux.