[SPARK-8662] SparkR Update SparkSQL Test

Test `infer_type` using a more fine-grained approach rather than comparing environments. Since `all.equal`'s behavior has changed in R 3.2, the test became unpassable.

JIRA here:
https://issues.apache.org/jira/browse/SPARK-8662

Author: cafreeman <cfreeman@alteryx.com>

Closes #7045 from cafreeman/R32_Test and squashes the following commits:

b97cc52 [cafreeman] Add `checkStructField` utility
3381e5c [cafreeman] Update SparkSQL Test