fixed flaky integration test on dynamic record sizes (#2292)
fixed flaky integration test on dynamic record sizes (#2292)

* fixed flaky integration test on dynamic record sizes

* small tweaks from CR, added documentation

* added small comment to threshold

* merge and added MTU_bytes constant
PKRoma/s2n,awslabs/s2n,awslabs/s2n,wcs1only/s2n,wcs1only/s2n,awslabs/s2n,PKRoma/s2n,wcs1only/s2n,PKRoma/s2n,PKRoma/s2n,awslabs/s2n,PKRoma/s2n,PKRoma/s2n,awslabs/s2n,wcs1only/s2n,wcs1only/s2n,wcs1only/s2n,wcs1only/s2n,PKRoma/s2n,awslabs/s2n,PKRoma/s2n,wcs1only/s2n