Added test when TTL has expired
[HttpKernel] fixed default TTL not applied under certain conditions
bug #10896 [HttpKernel] Fixed cache behavior when TTL has expired and a default "global" TTL is defined (alquerci, fabpot)

This PR was merged into the 2.3 branch.

Discussion
----------

[HttpKernel] Fixed cache behavior when TTL has expired and a default "global" TTL is defined

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #8232, #10822, #9919
| License       | MIT

From #9919:

"When the cache is stale the `validate` method `forward` the request to the backend. A new response will be created with or without TTL configuration. If the TTL was not set then the default one should be set like in the `fetch` method."

This PR fixes this issue, the tests provided in #9919 pass, and I've tweaked them to avoid the costly sleep calls.

Commits
-------

e3983e8 [HttpKernel] fixed default TTL not applied under certain conditions
bc42dae Added test when TTL has expired