Added delta for Request comparison

Sometimes, tests are failing because REQUEST_TIME
changes between the creation of the various sub-requests.

By using delta, we allow maximum of second of difference.

Example: https://travis-ci.org/symfony/symfony/jobs/17668158#L511
minor #10143 [Tests] [HttpKernel] Added delta for Request comparison (lavoiesl)

This PR was submitted for the 2.3-dev branch but it was merged into the 2.3 branch instead (closes #10143).

Discussion
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[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

Commits
-------

3a67309 [Tests] [HttpKernel] Added delta for Request comparison