tests-mbed_drivers-rtc: Add one second tolerance in functional tests.
tests-mbed_drivers-rtc: Add one second tolerance in functional tests.

Currently test assumes that 1 sec is long enough to set RTC time and read same time which has been set.
In some cases extra time for synchronisation between clock domains is needed and after setting/reading operations the read value might be different than one which has been set (+1 sec).
Additionally in some cases when lp ticker is based on RTC, the RTC implementation may use mechanism to trace elapsed seconds without modifying RTC registers. In such case it is possible that second will change immediately after setting time.

Add 1 sec tolerance (min possible) for such checks.

mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,betzw/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,mbedmicro/mbed,betzw/mbed-os,kjbracey-arm/mbed,andcor02/mbed-os,betzw/mbed-os,andcor02/mbed-os,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os,betzw/mbed-os,betzw/mbed-os,andcor02/mbed-os,betzw/mbed-os,andcor02/mbed-os,c1728p9/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,c1728p9/mbed-os,andcor02/mbed-os