systimer tests: do not require lp ticker
systimer tests: do not require lp ticker

SysTimer can run on us ticker now. Only deep sleep test
strictly requires lp ticker and it already has build conditions
in place.

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