grantleeheaderstyletest passes all tests in the KDE PIM Docker environment,
but fails in CI because it does not find an expected Spamassassin header
in some HTML-formatted output. It seems likely that the style cannot process
the message's spam headers because the antispam plugins are not installed
in the test environment.

This patch removes spam headers from the mbox file and the expected HTML
output, so the test will behave the same in the KDE PIM Docker and the
CI test environment.

I think it would still be a good idea to test that spam headers are
formatted correctly, so I will submit a companion patch that performs the
test in kdepim-addons, where the antispam plugins will be present.