Fix unittest.exp failure due to 'set debuginfod' addition

Fix unittest.exp failure due to 'set debuginfod' addition

The 'set debuginfod' change caused a regression in unittest.exp:

    Running selftest help_doc_invariants.
    help doc broken invariant: command 'info set debuginfod' help doc first line is not terminated with a '.' character
    help doc broken invariant: command 'set debuginfod' help doc first line is not terminated with a '.' character
    help doc broken invariant: command 'show debuginfod' help doc first line is not terminated with a '.' character
    Self test failed: self-test failed at ../../binutils-gdb/gdb/unittests/command-def-selftests.c:100

This patch fixes the problem.  I'm checking it in.