[SPARK-16940][SQL] `checkAnswer` should raise `TestFailedException` for wrong results

## What changes were proposed in this pull request?

This PR fixes the following to make `checkAnswer` raise `TestFailedException` again instead of `java.util.NoSuchElementException: key not found: TZ` in the environments without `TZ` variable. Also, this PR adds `QueryTestSuite` class for testing `QueryTest` itself.

```scala
- |Timezone Env: ${sys.env("TZ")}
+ |Timezone Env: ${sys.env.getOrElse("TZ", "")}
```

## How was this patch tested?

Pass the Jenkins tests with a new test suite.

Author: Dongjoon Hyun <dongjoon@apache.org>

Closes #14528 from dongjoon-hyun/SPARK-16940.

sql/core/src/test/scala/org/apache/spark/sql/QueryTest.scala

@@ -402,7 +402,7 @@ object QueryTest {
         s"""
         |Results do not match for query:
         |Timezone: ${TimeZone.getDefault}
-        |Timezone Env: ${sys.env("TZ")}
+        |Timezone Env: ${sys.env.getOrElse("TZ", "")}
         |
         |${df.queryExecution}
         |== Results ==
@@ -483,3 +483,11 @@ object QueryTest {
     }
   }
 }
+
+class QueryTestSuite extends QueryTest with test.SharedSQLContext {
+  test("SPARK-16940: checkAnswer should raise TestFailedException for wrong results") {
+    intercept[org.scalatest.exceptions.TestFailedException] {
+      checkAnswer(sql("SELECT 1"), Row(2) :: Nil)
+    }
+  }
+}