[SPARK-12168][SPARKR] Add automated tests for conflicted function in R

Currently this is reported when loading the SparkR package in R (probably would add is.nan)
```
Loading required package: methods

Attaching package: ‘SparkR’

The following objects are masked from ‘package:stats’:

    cov, filter, lag, na.omit, predict, sd, var

The following objects are masked from ‘package:base’:

    colnames, colnames<-, intersect, rank, rbind, sample, subset,
    summary, table, transform
```

Adding this test adds an automated way to track changes to masked method.
Also, the second part of this test check for those functions that would not be accessible without namespace/package prefix.

Incidentally, this might point to how we would fix those inaccessible functions in base or stats.
Looking for feedback for adding this test.

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #10171 from felixcheung/rmaskedtest.