[gdb] Add unit test for gdb::sequential_for_each

[gdb] Add unit test for gdb::sequential_for_each

With commit 18a5766d09c ("[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.