Allow Staking tests to run with session length other than 1 (#7719)

* fix periodic session

* Allow staking tests to run with session lengths other than 1.

* Update frame/staking/src/mock.rs

Co-authored-by: Guillaume Thiolliere <[email protected]>

* Fix all tests with session length 5.

* Test for active != current

* Better doc

* Update frame/staking/src/lib.rs

Co-authored-by: Guillaume Thiolliere <[email protected]>

* also set the timestamp properly.

* trigger CI

* Revert "trigger CI"

This reverts commit 0f254944cdad848aa6e63bd8a618db95447a8e68.

* Update frame/staking/src/lib.rs

Co-authored-by: Guillaume Thiolliere <[email protected]>