tests/kernel/usage/thread_runtime_stats: relax precision test for RISC-V

This commit relaxes the precision requirements for idle event statistic
test when RISC-V machine timer driver is used. This is needed for some
platforms (e.g. hifive1), for which the cycle count is too low to pass
the checks where a percent deviation of peak cycles count is allowed.

Signed-off-by: Filip Kokosinski <fkokosinski@antmicro.com>