Improve test coverage for ApiBase.php

Improve test coverage for ApiBase.php

One bug fixed: a timestamp of '00' or similar would get interpreted as
'now' by mistake instead of Unix timestamp 0, without throwing the
warning for using 0 instead of 'now'.  This is because it called
wfTimestamp() once to parse the input date, got a Unix timestamp of 0
back, and then tried passing that 0 back to wfTimestamp again to
reformat as a wiki date, but it got reinterpreted as 'now'.

Also fixed parameters with type "user" to validate usernames more
correctly.  This might be risky, though, if I missed any valid
usernames, or if API clients were for some reason relying on passing in
invalid usernames.  If we don't actually want to do this, we should add
a comment explaining why we're allowing any title without a fragment
rather than validating properly.

Still lots more work to do here.

Change-Id: I56b4290263df8698efdbddda71a7eabd9e303abc