Minor - code householding (tests)

- Remove redundant uses of WrapperMock
- Use assertThat() from JUnit, not hamcrest
- Use hamcrest Matchers everywhere (not BaseMatchers etc.)
- Favor Mockito's argThat() over using ArgumentCaptor (more succinct)
- Delete useless test classes

src/test/java/fr/xephi/authme/datasource/FlatFileIntegrationTest.java

@@ -16,8 +16,8 @@ import java.util.List;
 import static fr.xephi.authme.AuthMeMatchers.equalToHash;
 import static fr.xephi.authme.AuthMeMatchers.hasAuthBasicData;
 import static fr.xephi.authme.AuthMeMatchers.hasAuthLocation;
-import static org.hamcrest.CoreMatchers.hasItem;
 import static org.hamcrest.Matchers.equalTo;
+import static org.hamcrest.Matchers.hasItem;
 import static org.hamcrest.Matchers.hasSize;
 import static org.junit.Assert.assertThat;