gdb/testsuite: Remove duplicates from gdb.base/stack-checking.exp
When running the testsuite I have:
Running .../gdb/testsuite/gdb.base/stack-checking.exp ...
DUPLICATE: gdb.base/stack-checking.exp: bt
DUPLICATE: gdb.base/stack-checking.exp: bt
Fix by using with_test_prefix.
Tested on x86_64-linux.