With commit 18a5766d09 ("[gdbsupport] Add sequential_for_each") I added a
drop-in replacement for gdb::parallel_for_each, but there's nothing making
sure that the two remain in sync.
Extend the unit test for gdb::parallel_for_each to test both.
Do this using a slightly unusual file-self-inclusion. Doing so keep things
readable and maintainable, and avoids macrofying functions.
Tested on x86_64-linux.