Fix no-standalone-expect false positive in helper methods

This allows code such as the following to pass:

```
export class PageObjectModel {
  foo(x) {
    expect(x).toBeVisible();
  }
}
```

src/rules/no-standalone-expect.ts

@@ -29,7 +29,7 @@ const getBlockType = (
     const expr = func.parent;
 
     // arrow function or function expr
-    if (expr.type === 'VariableDeclarator') {
+    if (expr.type === 'VariableDeclarator' || expr.type === 'MethodDefinition') {
       return 'function';
     }
 

test/spec/no-standalone-expect.spec.ts

@@ -74,6 +74,8 @@ runRuleTester('no-standalone-expect', rule, {
     '{}',
     'test.only("an only", value => { expect(value).toBe(true); });',
     'test.concurrent("an concurrent", value => { expect(value).toBe(true); });',
+    'class Helper { foo() { expect(1).toBe(1); } }',
+    'class Helper { foo = () => { expect(1).toBe(1); } }',
     // Global aliases
     {
       code: dedent`