session_ info: small fixes (#2106)

* session_info: use proper timeout in test

* Revert "fix off-by-one error"

This reverts commit 35cb56305a19134acd8a8f881f3aabf999a09d74.

* session_info: use correct EarliestStoredSession when introduced on a live chain

* use saturating_sub

Co-authored-by: Bastian Köcher <[email protected]>

* session_info: revert the timeout test

* session_info: rust is dumb

Co-authored-by: Bastian Köcher <[email protected]>