[SPARK-9121] [SPARKR] Get rid of the warnings about `no visible global...

[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

dev/lint-r.R

@@ -15,15 +15,21 @@
 # limitations under the License.
 #
 
+argv <- commandArgs(TRUE)
+SPARK_ROOT_DIR <- as.character(argv[1])
+
 # Installs lintr from Github.
 # NOTE: The CRAN's version is too old to adapt to our rules.
 if ("lintr" %in% row.names(installed.packages())  == FALSE) {
   devtools::install_github("jimhester/lintr")
 }
-library(lintr)
 
-argv <- commandArgs(TRUE)
-SPARK_ROOT_DIR <- as.character(argv[1])
+library(lintr)
+library(methods)
+library(testthat)
+if (! library(SparkR, lib.loc = file.path(SPARK_ROOT_DIR, "R", "lib"), logical.return = TRUE)) {
+  stop("You should install SparkR in a local directory with `R/install-dev.sh`.")
+}
 
 path.to.package <- file.path(SPARK_ROOT_DIR, "R", "pkg")
 lint_package(path.to.package, cache = FALSE)