Many compiler checks rely on optimizations being enabled, so we want
these to show up in CI. Use RelWithDebInfo instead of Release so that
assert() statements aren't compiled out.
Adapt some tests for OpenBSD:
- scrollback_spec:
- seq(1) is not available on OpenBSD: we'd use jot(1).
- Instead use a (hopefully) portable awk(1) snippet.
- channels_spec
- job_spec
- tui_spec
