[SPARK-17674][SPARKR] check for warning in test output

[felixcheung]

What changes were proposed in this pull request?

testthat library we are using for testing R is redirecting warning (and disabling options("warn" = 2)), we need to have a way to detect any new warning and fail

How was this patch tested?

manual testing, Jenkins

[SparkQA]

Test build #67308 has finished for PR 15576 at commit 5185a37.

• This patch fails SparkR unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[felixcheung]

Test failure is intentional, it's picking up the following warnings:

Warnings -----------------------------------------------------------------------
1. createDataFrame uses files for large objects (@test_sparkSQL.R#215) - Use Sepal_Length instead of Sepal.Length  as column name

2. createDataFrame uses files for large objects (@test_sparkSQL.R#215) - Use Sepal_Width instead of Sepal.Width  as column name

3. createDataFrame uses files for large objects (@test_sparkSQL.R#215) - Use Petal_Length instead of Petal.Length  as column name

4. createDataFrame uses files for large objects (@test_sparkSQL.R#215) - Use Petal_Width instead of Petal.Width  as column name

Which is fixed in PR #15560

[felixcheung]

Rebased. This should pass now.

[SparkQA]

Test build #67321 has finished for PR 15576 at commit 1c9bd2e.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[wangmiao1981]

Thanks for fixing this! I encountered this issue before.

[felixcheung]

merged to master, thanks