fix: mock ApiTracerFactory.withContext()

Author: diegomarquezp

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/stub/SkipTrailersTest.java

@@ -95,6 +95,7 @@ public void setUp() throws Exception {
     server = FakeServiceBuilder.create(hackedService).start();
 
     when(tracerFactory.newTracer(Mockito.any(), Mockito.any(), Mockito.any())).thenReturn(tracer);
+    when(tracerFactory.withContext(Mockito.any())).thenReturn(tracerFactory);
 
     BigtableDataSettings.Builder clientBuilder =
         BigtableDataSettings.newBuilderForEmulator(server.getPort())