fix(no-export): treat describe blocks as test files (#1978)

Report exports in files that only contain describe blocks, aligning
behavior with the rule documentation.

Author: eryue0220

src/rules/__tests__/no-export.test.ts

@@ -82,5 +82,15 @@ ruleTester.run('no-export', rule, {
       code: 'module.export.invalid = function() {}; ;  test("a test", () => { expect(1).toBe(1);});',
       errors: [{ endColumn: 22, column: 1, messageId: 'unexpectedExport' }],
     },
+    {
+      code: dedent`
+        module.exports = function () {};
+
+        describe('a test', () => {
+          expect(1).toBe(1);
+        });
+      `,
+      errors: [{ endColumn: 15, column: 1, messageId: 'unexpectedExport' }],
+    },
   ],
 });

src/rules/no-export.ts

@@ -33,7 +33,7 @@ export default createRule({
       },
 
       CallExpression(node) {
-        if (isTypeOfJestFnCall(node, context, ['test'])) {
+        if (isTypeOfJestFnCall(node, context, ['describe', 'test'])) {
           hasTestCase = true;
         }
       },