[FrameworkBundle] added WebTestHelper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10753
| License       | MIT

Added a way to perform functional test even when it is not possible to extends WebTestCase. `WebTestHelper::shutdownKernel()` must here be called by the user, so WebTestCase still remains easier to use when it is possible. WebTestHelper should be provided as an alternative.