Add unit test that excercises sampling in RFileScanner iterator.

[keith-turner]

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

    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 #5490 (comment)

[kevinrr888]

Relevant thread and PR: #5525 (comment)