Add extra test case for outcome printing of function parameter types (rescript-lang#460)

Author: IwanKaramazow

syntax/tests/oprint/expected/oprint.resi.txt

@@ -493,4 +493,5 @@ and ASet: {
 }
 type emptyObject = {.}
 let f: (~x: 'a=?, ~y: 'b) => option<'a>
-type call = CleanStart
+type call = CleanStart
+let f: (~a: int=?) => int

syntax/tests/oprint/oprint.res

@@ -338,3 +338,5 @@ type emptyObject = {.}
 let f = (~x=?, ~y as _) => x
 
 type call = CleanStart
+
+let f = (~a=1) => 1