[SPARK-11672][ML] flaky spark.ml read/write tests

We set `sqlContext = null` in `afterAll`. However, this doesn't change `SQLContext.activeContext`  and then `SQLContext.getOrCreate` might use the `SparkContext` from previous test suite and hence causes the error. This PR calls `clearActive` in `beforeAll` and `afterAll` to avoid using an old context from other test suites.

cc: yhuai

Author: Xiangrui Meng <meng@databricks.com>

Closes #9677 from mengxr/SPARK-11672.2.

mllib/src/test/java/org/apache/spark/ml/util/JavaDefaultReadWriteSuite.java

@@ -23,7 +23,7 @@ import java.io.IOException;
 import org.junit.After;
 import org.junit.Assert;
 import org.junit.Before;
-import org.junit.Ignore;
+import org.junit.Test;
 
 import org.apache.spark.api.java.JavaSparkContext;
 import org.apache.spark.sql.SQLContext;
@@ -50,7 +50,7 @@ public class JavaDefaultReadWriteSuite {
     Utils.deleteRecursively(tempDir);
   }
 
-  @Ignore // SPARK-11672
+  @Test
   public void testDefaultReadWrite() throws IOException {
     String uid = "my_params";
     MyParams instance = new MyParams(uid);

mllib/src/test/scala/org/apache/spark/ml/classification/LogisticRegressionSuite.scala

@@ -872,7 +872,7 @@ class LogisticRegressionSuite
     assert(model1a0.intercept ~== model1b.intercept absTol 1E-3)
   }
 
-  ignore("read/write") { // SPARK-11672
+  test("read/write") {
     // Set some Params to make sure set Params are serialized.
     val lr = new LogisticRegression()
       .setElasticNetParam(0.1)

mllib/src/test/scala/org/apache/spark/ml/feature/BinarizerSuite.scala

@@ -68,7 +68,7 @@ class BinarizerSuite extends SparkFunSuite with MLlibTestSparkContext with Defau
     }
   }
 
-  ignore("read/write") { // SPARK-11672
+  test("read/write") {
     val binarizer = new Binarizer()
       .setInputCol("feature")
       .setOutputCol("binarized_feature")

mllib/src/test/scala/org/apache/spark/ml/util/DefaultReadWriteTest.scala

@@ -105,7 +105,7 @@ object MyParams extends Readable[MyParams] {
 class DefaultReadWriteSuite extends SparkFunSuite with MLlibTestSparkContext
   with DefaultReadWriteTest {
 
-  ignore("default read/write") { // SPARK-11672
+  test("default read/write") {
     val myParams = new MyParams("my_params")
     testDefaultReadWrite(myParams)
   }

mllib/src/test/scala/org/apache/spark/mllib/util/MLlibTestSparkContext.scala

@@ -32,11 +32,13 @@ trait MLlibTestSparkContext extends BeforeAndAfterAll { self: Suite =>
       .setMaster("local[2]")
       .setAppName("MLlibUnitTest")
     sc = new SparkContext(conf)
+    SQLContext.clearActive()
     sqlContext = new SQLContext(sc)
   }
 
   override def afterAll() {
     sqlContext = null
+    SQLContext.clearActive()
     if (sc != null) {
       sc.stop()
     }