Add another test case

Author: cknitt

tests/build_tests/super_errors/expected/optional_record_field_missing_case_result.res.expected

@@ -0,0 +1,14 @@
+
+  Warning number 8
+  /.../fixtures/optional_record_field_missing_case_result.res:4:3-7:3
+
+  2 │ 
+  3 │ let f = v =>
+  4 │   switch v {
+  5 │   | {b: Ok(x)} => x
+  6 │   | {b: Error(y)} => y
+  7 │   }
+  8 │ 
+
+  You forgot to handle a possible case here, for example: 
+  | {b: ?None}

tests/build_tests/super_errors/fixtures/optional_record_field_missing_case_result.res

@@ -0,0 +1,7 @@
+type t = {b?: result<string, string>}
+
+let f = v =>
+  switch v {
+  | {b: Ok(x)} => x
+  | {b: Error(y)} => y
+  }