OCL: Fix accurency test for OpenCL matrix operations

1. Increase epsilon for `ConvertTo` (with current epsilon test not passed if `test_loop_times > 1`)
2. `SetTo` need CV_8UC1 mask only. 

check_regression=*OCL*ConvertTo*:*OCL*SetTo*
test_filter=*OCL*ConvertTo*:*OCL*SetTo*
build_examples=OFF
test_modules=core