Add test for User::isBlockedFrom

Add test for User::isBlockedFrom

In the complicated decision tree for checking if a blocked
user is blocked from their talk page, one situation was not
being tested: there's a partial block to the user talk
namespace, the block is flagged as allowing a user to edit
their talk page, BUT $wgBlockAllowsUTEdit is false. In this
circumstance, the user should be blocked from editing their
talk page, as outlined in T210475.

Also, fix whitespace and make messages clearer, since this
is now quite complicated.

Change-Id: I234f3019d55a6da0da091a2eaae6c791be01b436