[SPARK-34115][CORE] Check SPARK_TESTING as lazy val to avoid slowdown

### What changes were proposed in this pull request?
Check SPARK_TESTING as lazy val to avoid slow down when there are many environment variables

### Why are the changes needed?
If there are many environment variables, sys.env slows is very slow. As Utils.isTesting is called very often during Dataframe-Optimization, this can slow down evaluation very much.

An example for triggering the problem can be found in the bug ticket https://issues.apache.org/jira/browse/SPARK-34115

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
With the example provided in the ticket.

Closes apache#31244 from nob13/bug/34115.

Lead-authored-by: Norbert Schultz <norbert.schultz@reactivecore.de>
Co-authored-by: Norbert Schultz <noschultz@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
(cherry picked from commit c3d8352)
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

core/src/main/scala/org/apache/spark/util/Utils.scala

@@ -1879,7 +1879,9 @@ private[spark] object Utils extends Logging {
    * Indicates whether Spark is currently running unit tests.
    */
   def isTesting: Boolean = {
-    sys.env.contains("SPARK_TESTING") || sys.props.contains(IS_TESTING.key)
+    // Scala's `sys.env` creates a ton of garbage by constructing Scala immutable maps, so
+    // we directly use the Java APIs instead.
+    System.getenv("SPARK_TESTING") != null || System.getProperty(IS_TESTING.key) != null
   }
 
   /**