[SPARK-11967][SQL] Consistent use of varargs for multiple paths in DataFrameReader

[rxin]

This patch makes it consistent to use varargs in all DataFrameReader methods, including Parquet, JSON, text, and the generic load function.

Also added a few more API tests for the Java API.

[rxin]

cc @koertkuipers since you did the change for load(Array) - I'm changing it to varargs here.

[SparkQA]

Test build #46623 has finished for PR 9945 at commit 01897a8.

• This patch fails PySpark unit tests.
• This patch does not merge cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #46638 has finished for PR 9945 at commit bd9a538.

• This patch fails PySpark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * public final class SortedIterator extends UnsafeSorterIterator\n * case class GetStructField(child: Expression, ordinal: Int, name: Option[String] = None)\n * case class PrettyAttribute(name: String, dataType: DataType = NullType)\n

[SparkQA]

Test build #46641 has finished for PR 9945 at commit f97193c.

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

[SparkQA]

Test build #46642 has finished for PR 9945 at commit c824f24.

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

[AmplabJenkins]

Test FAILed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/46641/
Test FAILed.

[cloud-fan]

There is no @Test here, is it by means?

[rxin]

oh we should add it. can you add it in ur next pr?