test(validation): add OneOf variable tests from 16.x.x to next

- graphql#4363 and graphql#4453 on 16.x.x had some tests missing from graphql#4194 on next

Author: JoviDeCroock

src/validation/__tests__/VariablesInAllowedPositionRule-test.ts

@@ -493,3 +493,44 @@ describe('Validate: Variables are in allowed positions', () => {
     });
   });
 });
+
+describe('Validates OneOf Input Objects', () => {
+  it('Allows exactly one non-nullable variable', () => {
+    expectValid(`
+      query ($string: String!) {
+        complicatedArgs {
+          oneOfArgField(oneOfArg: { stringField: $string })
+        }
+      }
+    `);
+  });
+
+  it('Undefined variable in oneOf input object', () => {
+    expectErrors(`
+      {
+        complicatedArgs {
+          oneOfArgField(oneOfArg: { stringField: $undefinedVariable })
+        }
+      }
+    `).toDeepEqual([]);
+  });
+
+  it('Forbids one nullable variable', () => {
+    expectErrors(`
+      query ($string: String) {
+        complicatedArgs {
+          oneOfArgField(oneOfArg: { stringField: $string })
+        }
+      }
+    `).toDeepEqual([
+      {
+        message:
+          'Variable "$string" is of type "String" but must be non-nullable to be used for OneOf Input Object "OneOfInput".',
+        locations: [
+          { line: 2, column: 14 },
+          { line: 4, column: 50 },
+        ],
+      },
+    ]);
+  });
+});