@Test
  public void testGetExternalContext() {
    ExternalContext externalContext = mock(ExternalContext.class);
    Context context = buildWithExternalContext(externalContext);
    assertEquals(externalContext, context.getExternalContext());
  }
Given a concrete context, getExternalContext should return it.
samza-core/src/test/java/org/apache/samza/context/TestContextImpl.java