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.
1 file changed