parser: Update reference to PatchComment

parser: Update reference to PatchComment

Commit 0686a736fbf6d869bd31bd135ba38080ac96de22 split out 'CoverLetter'
from the old 'Submission' model, removing the common 'Comment' model in
favour of distinct 'CoverComment' and 'PatchComment' models in the
process. Unfortunately we misssed some references to the old model in
the 'patchwork.parser' module. Correct these now, adding unit tests to
prevent regressions.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Fixes: 0686a736 ("models: Split 'CoverLetter' from 'Submission'")
Closes: #384