gdb/testsuite: Remove duplicates from gdb.base/del.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/del.exp ...
    DUPLICATE: gdb.base/del.exp: info break after removing break on main

Refactor slightly this test to run the various configurations under
foreach_with_prefix so each variant is automatically prefixed, ensuring
that the forgotten custom test name cannot happen.

Tested on x86_64-linux.