OpenCL error checking + unittest is broken
Pending https://github.com/nim-lang/opencl/pull/3

The `check` defined in nim/opencl catches any expression and conflicts with check from CLBlast and nim cuda and also with check from `unittest`

```
Error: ambiguous call; both opencl.check(a: expr): typed[declared in /Users/tesuji/.nimble/pkgs/opencl-1.0/opencl.nim(191, 9)] and unittest.check(conditions: untyped)[declared in lib/pure/unittest.nim(563, 6)] match for: (TClResult)
```

Trying to use qualified call like `opencl.check` yields:

```
Error: type mismatch: got (proc (t: Tensor[opencl.T]): ClTensor[opencl.T], TClResult)
but expected one of:
template check(a: expr)
template check(status: CLBlastStatusCode)
macro check(conditions: untyped): untyped
```