[FLINK-11168][tests] Relax time-constraints for LargePlanTest

The test completes in 500ms on my machine. On Travis the resources are much
more limited and it fails occasionally.

The following should ensure the test does not fail anymore:

- Increase test timeout to 30 seconds
- Reduce complexity of tests by decreasing branching factor

flink-tests/src/test/java/org/apache/flink/test/planning/LargePlanTest.java

@@ -34,9 +34,9 @@
  */
 public class LargePlanTest {
 
-	@Test(expected = OptimizerPlanEnvironment.ProgramAbortException.class, timeout = 15_000)
+	@Test(expected = OptimizerPlanEnvironment.ProgramAbortException.class, timeout = 30_000)
 	public void testPlanningOfLargePlan() throws Exception {
-		runProgram(new PreviewPlanEnvironment(), 10, 50);
+		runProgram(new PreviewPlanEnvironment(), 10, 20);
 	}
 
 	private static void runProgram(ExecutionEnvironment env, int depth, int width) throws Exception {