@Test
  public void testGetKeyType_works() throws Exception {
    assertThat(manager.getKeyType())
        .isEqualTo("type.googleapis.com/google.crypto.tink.PrfBasedDeriverKey");
  }
We test newKeyData directly because PrfBasedDeriverKeyManager has its own implementation.
java_src/src/test/java/com/google/crypto/tink/keyderivation/internal/PrfBasedDeriverKeyManagerTest.java