gdb/testsuite: Remove duplicates from gdb.base/pie-fork.exp

When running the testsuite, I have:

    Running .../gdb/testsuite/gdb.base/pie-fork.exp ...
    DUPLICATE: gdb.base/pie-fork.exp: test_no_detach_on_fork: continue

Fix by giving explicit names to the 'continue' commands that cause the
duplicate message.

Tested on x86_64-linux.