Well-formedness checker is overzealous about testing-expressions on the same line as `...` expressions

[blerner]

The following check-block gives a well-formedness error:

check:
  ... 6 ... is 6
end

The same error occurs even if the testing expression is not in a check-block:

fun f():
  ... 6 ... is 6
end

(N.B. It does also give the correct error that the test expression is not in a check block; that's working properly.)