[SPARK-20105][TESTS][R] Add tests for checkType and type string in structField in R

## What changes were proposed in this pull request?

It seems `checkType` and the type string in `structField` are not being tested closely. This string format currently seems SparkR-specific (see https://github.com/apache/spark/blob/d1f6c64c4b763c05d6d79ae5497f298dc3835f3e/sql/core/src/main/scala/org/apache/spark/sql/api/r/SQLUtils.scala#L93-L131) but resembles SQL type definition.

Therefore, it seems nicer if we test positive/negative cases in R side.

## How was this patch tested?

Unit tests in `test_sparkSQL.R`.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17439 from HyukjinKwon/r-typestring-tests.