Fix undefined behavior in case of time overflow on 32-bit platforms

[d06alexandrov]

Fixes #1762

Currently signed long int is used for storing current time in milliseconds (time is calculated by GetPlatformSpecificTimeInMillis()).
But on 32-bit platforms long could be a 32-bit number (e.g. C17 standard declares at 5.2.4.2.1 that long int should be at least 32 bits long).

Current unix timestamp in seconds is 31 bit long and timestamp in milliseconds is at least 41 bit long.

C/C++ standards declare that behavior of signed integer overflow is undefined (e.g. C17 example at 3.4.3).

The platform specific time is used only for calculating difference between end and start time. So, long could be replaced with unsigned long in that case without any negative effects.

[coveralls]

coverage: 99.323%. remained the same
when pulling 0ade506 on d06alexandrov:fix_time_ub
into c3625dc on cpputest:master.