chore(spanner-jdbc): disable flaky IsolationLevelTest.testIsolationLevel (#14859)

Disable flaky
`com.example.spanner.jdbc.IsolationLevelTest.testIsolationLevel`.

b/579438023

Author: jinseopkim0

java-spanner-jdbc/samples/snippets/src/test/java/com/example/spanner/jdbc/IsolationLevelTest.java

@@ -26,6 +26,7 @@
 import java.util.Properties;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
+import org.junit.Ignore;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -89,6 +90,7 @@ public static void stopEmulator() {
     }
   }
 
+  @Ignore("Flaky test: b/579438023")
   @Test
   public void testIsolationLevel() throws SQLException {
     isolationLevel("emulator-project", "my-instance", "my-database", PROPERTIES);