add test for rescript-lang#5359

Author: jchavarri

jscomp/build_tests/super_errors/expected/warnings4.res.expected

@@ -0,0 +1,13 @@
+
+ Warning number 8
+ /.../fixtures/warnings4.res:11:1-13:1
+
+ 9 │ @val external x: myType = "myVariable"
+ 10 │ 
+ 11 │ switch x {
+ 12 │  | #first => Js.log("first")
+ 13 │ }
+ 14 │ 
+
+ You forgot to handle a possible case here, for example: 
+ (#second|#fourth|#third)

jscomp/build_tests/super_errors/fixtures/warnings4.res

@@ -0,0 +1,13 @@
+type myType = [
+ | #first
+ | #second
+ | #third
+ | #fourth
+]
+
+// just a quick way to make the switch compile
+@val external x: myType = "myVariable"
+
+switch x {
+ | #first => Js.log("first")
+}