[gdb/unittests] Ignore spellcheck warning in rsp-low-selftests.c

Ignore the following spellcheck warning:
...
$ codespell --config gdb/contrib/setup.cfg gdb/unittests
gdb/unittests/rsp-low-selftests.c:54: fo ==> of, for, to, do, go
...
and add gdb/unittests to the pre-commit codespell configuration.

Approved-By: Tom Tromey <tom@tromey.com>