test: Fix date in test message in multipart test.

The test message date, "Tue, 05 Jan 2001 15:43:57 -0000", is not
actually a real date.  05 Jan 2001 was in fact a Friday, not a
Tuesday.  Date parsers (such as "date" in coreutils) will return "Fri"
as the day for this string, even if "Tue" is specified.

Also, the time zone "-0000" is actually always returned as "+0000", so
we change that here was well.

This will be relevant for later patches when we begin parsing rfc822
part headers, where gmime returns a parsed date string.

If we do want to test date parsing, we should do that in a separate
test.