OCL: Increase epsilon for OpenCL version UMatDot and ReduceAvg accurency tests

This epsilon allows tests to pass with `test_loop_times=30`

check_regression=*OCL*UMatDot*:*OCL*ReduceAvg*
test_filter=*OCL*UMatDot*:*OCL*ReduceAvg*
build_examples=OFF
test_modules=core