tests: kernel: thread_runtime_stats: Relax precision test for QEMU

This commit relaxes the idle event statistics test precision requirement
for emulated QEMU targets because the cycle counts may be inaccurate when
the host CPU is overloaded (e.g. when running tests with twister) and a
high failure rate is observed for this test in the CI.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>