[SPARK-9121] [SPARKR] Get rid of the warnings about `no visible global function definition` in SparkR

[[SPARK-9121] Get rid of the warnings about `no visible global function definition` in SparkR - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-9121)

## The Result of `dev/lint-r`
[The result of lint-r for SPARK-9121 at the revision:1ddd0f2f when I have sent a PR](https://gist.github.com/yu-iskw/6f55953425901725edf6)

Author: Yu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #7567 from yu-iskw/SPARK-9121 and squashes the following commits:

c8cfd63 [Yu ISHIKAWA] Fix the typo
b1f19ed [Yu ISHIKAWA] Add a validate statement for local SparkR
1a03987 [Yu ISHIKAWA] Load the `testthat` package in `dev/lint-r.R`, instead of using the full path of function.
3a5e0ab [Yu ISHIKAWA] [SPARK-9121][SparkR] Get rid of the warnings about `no visible global function definition` in SparkR