[DNM] Try to fix the ambiguous use of expect on Swift 5.3

[ikesyo]

ref: #809

[ikesyo]

@ashdnazg

I think a simpler way would be to just wrap the 2nd except with #if !swift(>=5.3).

That doesn't work. In that case expect { 1 } is evaluated as Expectation<() -> Int> (T is () -> Int, not Int: a trailing closure itself is wrapped by @autoclosure). What we want here is Expectation<Int>.

[ikesyo]

Superseded by #824 and #825.