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/security/HashUtilsTest.java

@@ -6,9 +6,9 @@ import java.security.MessageDigest;
 import java.util.ArrayList;
 import java.util.List;
 
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.contains;
 import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
 
 /**
  * Test for {@link HashUtils}.

src/test/java/fr/xephi/authme/security/RandomStringTest.java

@@ -4,8 +4,8 @@ import org.junit.Test;
 
 import java.util.regex.Pattern;
 
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
 
 /**
  * Test for {@link RandomString}.