Remove use of junit.framework package

There are certain places in our code base where we use classes from the
`junit.framework` package.

The `junit.framework` package exists for backwards-compatibility with
JUnit 3[1]. We are not a legacy code base stuck on JUnit 3, thus we
should use the JUnit 4 API.

Replace uses of the `junit.framework` package with the JUnit 4 API
equivalents, namely:

 * Rather than using `junit.framework.TestCase.assertNotNull`, use the
   JUnit 4 API's `org.junit.Assert.assertNotNull`[2].

 * Rather than using `junit.framework.AssertionFailedError`, use
   `java.lang.AssertionError` instead, since that is what is thrown by
   the JUnit 4 API on assertion failures.

[1] From the package-info.java of `junit.framework` at
    https://github.com/junit-team/junit4/blob/02c328028b4d32c15bbf0becc9838e54ecbafcbf/src/main/java/junit/framework/package-info.java#L2

    "Provides JUnit v3.x core classes"

[2] https://stackoverflow.com/a/291074

    "JUnit 3.X: junit.framework.Assert

     JUnit 4.X: org.junit.Assert

     Prefer the newest one, especially when running JDK5 and higher with
     annotation support."

src/test/java/seedu/address/storage/StorageManagerTest.java

@@ -1,7 +1,7 @@
 package seedu.address.storage;
 
-import static junit.framework.TestCase.assertNotNull;
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertTrue;
 import static seedu.address.testutil.TypicalPersons.getTypicalAddressBook;
 

src/test/java/seedu/address/testutil/Assert.java

@@ -1,7 +1,5 @@
 package seedu.address.testutil;
 
-import junit.framework.AssertionFailedError;
-
 /**
  * A set of assertion methods useful for writing tests.
  */
@@ -37,10 +35,10 @@ public static void assertThrows(Class<? extends Throwable> expectedException, St
                 return;
             }
 
-            throw new AssertionFailedError(errorMessage);
+            throw new AssertionError(errorMessage);
         }
 
-        throw new AssertionFailedError(String.format(
+        throw new AssertionError(String.format(
                 "Expected %s to be thrown, but nothing was thrown.", expectedException.getName()));
     }