Add unit test that excercises sampling in RFileScanner iterator.

The following seems good for ClientSideIteratorScanner and is tested in `SampleIT.testIterator()`.

```suggestion
if (samplerConfig.isEmpty()) {
throw new SampleNotPresentException();
}
```

Inspecting table props seems like the right way to go, however the bit of code here is not complete for that. Looking into RFileScanner it may benefit from a secondary check of tables props. AFIACT there are no tests for calling `cloneWithSamplingEnabled()` when using RFileScanner. In a follow on PR I could add unit test that cover calling this code when using RFileScanner and maybe add some code that inspects the table config as a secondary step when samplerConfig.isEmpty().

_Originally posted by @keith-turner in https://github.com/apache/accumulo/pull/5490#discussion_r2054898012_