Summary:
Fix the test:
* Use kTimeMultiplier to give *SAN tests more time.
* Check the amount of time passed since the last update before checking the value expected.

Test Plan: ./yb_build.sh tsan --cxx-test 'TEST_F(PgMiniTest, FollowerReads) {' --clang17 -n 100

Reviewers: pjain, rthallam

Reviewed By: pjain, rthallam

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D37832