Replace REQUEST_TIME in Functional and FunctionalJavascript tests

Problem/Motivation

REQUEST_TIME is a deprecated global. There are several issues to remove it's use in core.

This issue is to remove it from just Functional and FunctionalJavascript tests.

Proposed resolution

The Symfony Request object will always a value for server's REQUEST_TIME, so we can use the one created in FunctionalTestSetupTrait::prepareEnvironment(), during the test's setUp, as a replacement for the constant.

Remaining tasks

Check that the unresolved issues in the first MR are resolved.
Update the latest MR, the diff no longer applies.

User interface changes

API changes

Data model changes

Release notes snippet