send-email: fix broken transferEncoding tests

send-email: fix broken transferEncoding tests

I fixed a bug that had broken the reading of sendmail.transferEncoding
in 3494dfd3ee ("send-email: do defaults -> config -> getopt in that
order", 2019-05-09), but the test I added in that commit did nothing
to assert the bug had been fixed.

That issue originates in 8d81408435 ("git-send-email: add
--transfer-encoding option", 2014-11-25) which first added the
"sendemail.transferencoding=8bit".

That test has never done anything meaningful. It tested that the
"--transfer-encoding=8bit" option would turn on the 8bit
Transfer-Encoding, but that was the default at the time (and now). As
checking out 8d81408435 and editing the test to remove that option
will reveal, supplying it never did anything.

So when I copied it thinking it would work in 3494dfd3ee I copied a
previously broken test, although I was making sure it did the right
thing via da-hoc debugger inspection, so the bug was fixed.

So fix the test I added in 3494dfd3ee, as well as the long-standing
test added in 8d81408435. To test if we're actually setting the
Transfer-Encoding let's set it to 7bit, not 8bit, as 7bit will error
out on "email-using-8bit".

This means that we can remove the "sendemail.transferencoding=7bit
fails on 8bit data" test, since it was redundant, we now have other
tests that assert that that'll fail.

While I'm at it convert "git config <key> <value>" in the test setup
to just "-c <key>=<value>" on the command-line. Then we don't need to
cleanup after these tests, and there's no sense in asserting where
config values come from in these tests, we can take that as a given.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>