core: Use real ByteBuffer instead of mock (grpc#9523)

My motivation for making this change is that [`ByteBuffer` is becoming
`sealed`](https://download.java.net/java/early_access/loom/docs/api/java.base/java/nio/ByteBuffer.html)
in new versions of Java. This makes it impossible for Mockito's
_current_ default mockmaker to mock it.

That said, Mockito will likely [switch its default
mockmaker](mockito/mockito#2589) to an
alternative that _is_ able to mock `sealed` classes. However, there are
downside to that, such as [slower
performance](mockito/mockito#2589 (comment)),
so it's probably better to leave our options open by avoiding mocking at
all.

And in this case, it's equally easy to use real objects.

As a bonus, I think that real objects makes the code a little easier to
follow: Before, we created mocks that the code under test never
interacted with in any way. (The code just passed them through to a
delegate.) When I first read the tests, I was confused, since I assumed
that the mock we were creating was the same mock that we then passed to
`verify` at the end of the method. That turned out not to be the case.

core/src/test/java/io/grpc/internal/ForwardingReadableBufferTest.java

@@ -36,13 +36,10 @@
 import org.mockito.junit.MockitoJUnit;
 import org.mockito.junit.MockitoRule;
 
-/**
- * Tests for {@link ForwardingReadableBuffer}.
- */
+/** Tests for {@link ForwardingReadableBuffer}. */
 @RunWith(JUnit4.class)
 public class ForwardingReadableBufferTest {
-  @Rule
-  public final MockitoRule mocks = MockitoJUnit.rule();
+  @Rule public final MockitoRule mocks = MockitoJUnit.rule();
 
   @Mock private ReadableBuffer delegate;
   private ForwardingReadableBuffer buffer;
@@ -55,10 +52,7 @@ public void setUp() {
   @Test
   public void allMethodsForwarded() throws Exception {
     ForwardingTestUtil.testMethodsForwarded(
-        ReadableBuffer.class,
-        delegate,
-        buffer,
-        Collections.<Method>emptyList());
+        ReadableBuffer.class, delegate, buffer, Collections.<Method>emptyList());
   }
 
   @Test
@@ -99,7 +93,7 @@ public void readBytes() {
 
   @Test
   public void readBytes_overload1() {
-    ByteBuffer dest = mock(ByteBuffer.class);
+    ByteBuffer dest = ByteBuffer.allocate(0);
     buffer.readBytes(dest);
 
     verify(delegate).readBytes(dest);