[SPARK-3444] [CORE] Restore INFO level after log4j test.

Otherwise other tests don't log anything useful...

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes apache#7140 from vanzin/SPARK-3444 and squashes the following commits:

de14836 [Marcelo Vanzin] Better fix.
6cff13a [Marcelo Vanzin] [SPARK-3444] [core] Restore INFO level after log4j test.

core/src/test/scala/org/apache/spark/util/UtilsSuite.scala

@@ -486,11 +486,17 @@ class UtilsSuite extends SparkFunSuite with ResetSystemProperties with Logging {
 
   // Test for using the util function to change our log levels.
   test("log4j log level change") {
-    Utils.setLogLevel(org.apache.log4j.Level.ALL)
-    assert(log.isInfoEnabled())
-    Utils.setLogLevel(org.apache.log4j.Level.ERROR)
-    assert(!log.isInfoEnabled())
-    assert(log.isErrorEnabled())
+    val current = org.apache.log4j.Logger.getRootLogger().getLevel()
+    try {
+      Utils.setLogLevel(org.apache.log4j.Level.ALL)
+      assert(log.isInfoEnabled())
+      Utils.setLogLevel(org.apache.log4j.Level.ERROR)
+      assert(!log.isInfoEnabled())
+      assert(log.isErrorEnabled())
+    } finally {
+      // Best effort at undoing changes this test made.
+      Utils.setLogLevel(current)
+    }
   }
 
   test("deleteRecursively") {