[TEST] Different behaviors of SparkContext Conf when building SparkSession

## What changes were proposed in this pull request?
If the created ACTIVE sparkContext is not EXPLICITLY passed through the Builder's API `sparkContext()`, the conf of this sparkContext will also contain the conf set through the API `config()`; otherwise, the conf of this sparkContext will NOT contain the conf set through the API `config()`

## How was this patch tested?
N/A

Author: gatorsmile <gatorsmile@gmail.com>

Closes #18517 from gatorsmile/fixTestCase2.

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

@@ -98,12 +98,31 @@ class SparkSessionBuilderSuite extends SparkFunSuite {
     val session = SparkSession.builder().config("key2", "value2").getOrCreate()
     assert(session.conf.get("key1") == "value1")
     assert(session.conf.get("key2") == "value2")
+    assert(session.sparkContext == sparkContext2)
     assert(session.sparkContext.conf.get("key1") == "value1")
+    // If the created sparkContext is not passed through the Builder's API sparkContext,
+    // the conf of this sparkContext will also contain the conf set through the API config.
     assert(session.sparkContext.conf.get("key2") == "value2")
     assert(session.sparkContext.conf.get("spark.app.name") == "test")
     session.stop()
   }
 
+  test("create SparkContext first then pass context to SparkSession") {
+    sparkContext.stop()
+    val conf = new SparkConf().setAppName("test").setMaster("local").set("key1", "value1")
+    val newSC = new SparkContext(conf)
+    val session = SparkSession.builder().sparkContext(newSC).config("key2", "value2").getOrCreate()
+    assert(session.conf.get("key1") == "value1")
+    assert(session.conf.get("key2") == "value2")
+    assert(session.sparkContext == newSC)
+    assert(session.sparkContext.conf.get("key1") == "value1")
+    // If the created sparkContext is passed through the Builder's API sparkContext,
+    // the conf of this sparkContext will not contain the conf set through the API config.
+    assert(!session.sparkContext.conf.contains("key2"))
+    assert(session.sparkContext.conf.get("spark.app.name") == "test")
+    session.stop()
+  }
+
   test("SPARK-15887: hive-site.xml should be loaded") {
     val session = SparkSession.builder().master("local").getOrCreate()
     assert(session.sessionState.newHadoopConf().get("hive.in.test") == "true")