[Tests] [HttpKernel] Added delta for Request comparison

| Q             | A
| ------------- | ---
| Bug fix?      | yes (in tests only)
| New feature?  | -
| BC breaks?    | -
| Deprecations? | -
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Sometimes, tests are failing because REQUEST_TIME changes between the creation of the various sub-requests.

By using delta, we allow a maximum of one second of difference. 
Since this is a shady behaviour, I added minimal explanation as comments

Example: https://travis-ci.org/symfony/symfony/jobs/17668158#L511