Reject usernames with namespace or interwiki prefixes

Reject usernames with namespace or interwiki prefixes

User::getCanonicalName silently stripped namepace/iw prefixes,
resulting in weird behavior (e.g. entering 'Template:Foo' in the
registration form username field was accepted, but user 'Foo' was
created). Explicitly reject such names instead.

Also remove leading whitespace from test cases testing for something
else.

Bug: T94656
Change-Id: Idfb22f998cb069fca24abcf5809afd6e0324b0ae