[SPARK-3303][core] fix SparkContextSchedulerCreationSuite test error

run test with the master branch with this command when mesos native lib is set
sbt/sbt -Phive "test-only org.apache.spark.SparkContextSchedulerCreationSuite"

get this error:
[info] SparkContextSchedulerCreationSuite:
[info] - bad-master
[info] - local
[info] - local-*
[info] - local-n
[info] - local--n-failures
[info] - local-n-failures
[info] - bad-local-n
[info] - bad-local-n-failures
[info] - local-default-parallelism
[info] - simr
[info] - local-cluster
[info] - yarn-cluster
[info] - yarn-standalone
[info] - yarn-client
[info] - mesos fine-grained
[info] - mesos coarse-grained ** FAILED ***
[info] Executor Spark home `spark.mesos.executor.home` is not set!

Since `executorSparkHome` only used in `createCommand`, move `val executorSparkHome...` to `createCommand` to fix this issue.

Author: scwf <wangfei1@huawei.com>
Author: wangfei <wangfei_hello@126.com>

Closes #2199 from scwf/SparkContextSchedulerCreationSuite and squashes the following commits:

ef1de22 [scwf] fix code fomate
19d26f3 [scwf] fix conflict
d9a8a60 [wangfei] fix SparkContextSchedulerCreationSuite test error

core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/CoarseMesosSchedulerBackend.scala

@@ -71,11 +71,6 @@ private[spark] class CoarseMesosSchedulerBackend(
   val taskIdToSlaveId = new HashMap[Int, String]
   val failuresBySlaveId = new HashMap[String, Int] // How many times tasks on each slave failed
 
-  val executorSparkHome = conf.getOption("spark.mesos.executor.home")
-    .orElse(sc.getSparkHome())
-    .getOrElse {
-      throw new SparkException("Executor Spark home `spark.mesos.executor.home` is not set!")
-    }
 
   val extraCoresPerSlave = conf.getInt("spark.mesos.extra.cores", 0)
 
@@ -112,6 +107,11 @@ private[spark] class CoarseMesosSchedulerBackend(
   }
 
   def createCommand(offer: Offer, numCores: Int): CommandInfo = {
+    val executorSparkHome = conf.getOption("spark.mesos.executor.home")
+      .orElse(sc.getSparkHome())
+      .getOrElse {
+        throw new SparkException("Executor Spark home `spark.mesos.executor.home` is not set!")
+      }
     val environment = Environment.newBuilder()
     val extraClassPath = conf.getOption("spark.executor.extraClassPath")
     extraClassPath.foreach { cp =>