tests-events-queue: modify tolerance to percentage form
tests-events-queue: modify tolerance to percentage form

On some boards without SYSTICK support lp ticker is used to trace system ticks.
Event queue uses system ticker to count delay when callback is to be fired.
New ticker implementation for NRF51_DK provides fast and accurate counter for us ticker which shows inaccuracy of the system ticker(which is based on slow and inaccurate lp ticker). This is why measured error grows linearly and percentage form needs to be used.

Currently we use constant tolerance value equal to: 5 [ms].
This patch modifies tolerance to the following form: 5% of measured time [ms].

In this test the lowest delay is equal to 100 [ms], so 5% corresponds to 5 [ms] tolerance - original value. This means that min tolerance is 5 [ms] so this change should not have negative impact on other targets.

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