[gdb/testsuite] Fix gdbsever typo

I noticed a typo in the testsuite, twice: gdbsever.  Fix these.

Codespell doesn't detect it, so add a new file
gdb/contrib/codespell-dictionary.txt that contains a gdbsever->gdbserver
entry, and update gdb/contrib/setup.cfg to use it.

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