[test] Stop a test relying on inconsistent formatting of an error message.

[huonw]

Fixes rdar://problem/42351773.

[huonw]

@swift-ci please smoke test and merge

[jrose-apple]

You could use expected-error {{}} if you didn't want to change the test. Or just drop -verify altogether and use not.

[huonw]

I want to be specifically testing that the T type is the expected one inside the extension, and the error message was the first way I thought of doing that, but just writing the type seems better.

[huonw]

@swift-ci please smoke test Linux