[SPARK-12034][SPARKR] Eliminate warnings in SparkR test cases.

This PR:
1. Suppress all known warnings.
2. Cleanup test cases and fix some errors in test cases.
3. Fix errors in HiveContext related test cases. These test cases are actually not run previously due to a bug of creating TestHiveContext.
4. Support 'testthat' package version 0.11.0 which prefers that test cases be under 'tests/testthat'
5. Make sure the default Hadoop file system is local when running test cases.
6. Turn on warnings into errors.

Author: Sun Rui <rui.sun@intel.com>

Closes #10030 from sun-rui/SPARK-12034.