Adds minimal defaults for codegen so that those tests don't need to
specify the full command line.
Moves driver/testdata/compile to check/testdata/basics/raw_sem_ir, which
seems like it better reflects the focus. Removes the textual IR test
because we have plenty of those now.
Moves a multiline token diagnostic test to parse because (a) diagnostics
doesn't have other tests and (b) this is really testing the way that
parse structures the location.
Drops `--include-diagnostic-kind` from some driver tests that aren't
testing a diagnostic, so the flag felt a bit like noise (even before
this change, there are a few tests that don't specify it because they're
not intended to test an emitted diagnostic, just high-level diagnostic
behaviors).
Adds some comments to reflect my understanding of why a test exists,
when I was pausing to think about it.
Modifies the stdin test to do more dumps; which happens to expose we
currently misbehave.
Jon Ross-Perkins
806bee1063