test: skip SpanTest on JDK17 and higher (#1472)

The trick that the SpanTest class does to change a private static final field
is no longer possible in JDK12 and higher. This test therefore needs to be
skipped when the tests are running on Java12 or higher.

Fixes the build error in #1462

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SpanTest.java

@@ -40,6 +40,7 @@
 import java.util.concurrent.ScheduledThreadPoolExecutor;
 import org.junit.After;
 import org.junit.AfterClass;
+import org.junit.Assume;
 import org.junit.Before;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -99,8 +100,23 @@ public class SpanTest {
           .withDescription("Non-retryable test exception.")
           .asRuntimeException();
 
+  private static int getVersion() {
+    String version = System.getProperty("java.version");
+    if (version.startsWith("1.")) {
+      version = version.substring(2, 3);
+    } else {
+      int dot = version.indexOf(".");
+      if (dot != -1) {
+        version = version.substring(0, dot);
+      }
+    }
+    return Integer.parseInt(version);
+  }
+
   @BeforeClass
   public static void startStaticServer() throws Exception {
+    Assume.assumeTrue("This test is only supported on JDK11 and lower", getVersion() < 12);
+
     mockSpanner = new MockSpannerServiceImpl();
     mockSpanner.setAbortProbability(0.0D); // We don't want any unpredictable aborted transactions.
     mockSpanner.putStatementResult(StatementResult.update(UPDATE_STATEMENT, UPDATE_COUNT));
@@ -120,7 +136,8 @@ public static void startStaticServer() throws Exception {
             .start();
     channelProvider = LocalChannelProvider.create(uniqueName);
 
-    // Use a little bit reflection to set the test tracer.
+    // Use a little reflection to set the test tracer.
+    // This is not possible in Java 12 and later.
     java.lang.reflect.Field field = Tracing.class.getDeclaredField("traceComponent");
     field.setAccessible(true);
     java.lang.reflect.Field modifiersField =
@@ -133,8 +150,10 @@ public static void startStaticServer() throws Exception {
 
   @AfterClass
   public static void stopServer() throws InterruptedException {
-    server.shutdown();
-    server.awaitTermination();
+    if (server != null) {
+      server.shutdown();
+      server.awaitTermination();
+    }
   }
 
   @Before