Refresh TemplateParserJobTest::test_convertedHtml logic

Summary:
Use internal logic to parse the mail instead of triggering the parsing
externally and make it compatible with other tests.
TemplateParser differs between mMsg(write) and mOrigMsg (read).
If we use msg everywhere, this leads to asserts.

Reviewers: KDE PIM

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D20755