StringUtilTest: Remove Hamcrest String matcher

Currently, in StringUtilTest#getDetails_exceptionGiven, we use
Hamcrest's string matching and assert libraries.
This library was introduced in [1], however no justification was
given on why we specifically used these Hamcrest libraries. In fact,
this is the only instance in the entire project where we use this
dependency.

We are only using the assertThat and contains API of the Hamcrest
library, whose corresponding functionality is already implemented in
JUnit#assertTrue and Java String#contains. The only extra benefit that
this Hamcrest library provides is in its more detailed error message
when tests fail [2]. While convenient, this functionality is not
absolutely critical. This is because developers can use the debugger
to inspect the state of messages right before the error.

There is more value in this case to standardise the usage of our
assert statements. The Hamcrest library introduces a new API that
developers of AB4 will have to learn. New developers might be confused
why we use two different APIs to conduct tests. By standardising to
use the JUnit assertTrue, we remove the need to learn multiple APIs,
while achieving the same outcome.

Hence, let's remove the Hamcrest libraries and replace them with their
equivalent Java and JUnit APIs.

[1] 75942c9
[2] https://github.com/junit-team/junit4/wiki/matchers-and-assertthat

src/test/java/seedu/address/commons/util/StringUtilTest.java

@@ -1,7 +1,5 @@
 package seedu.address.commons.util;
 
-import static org.hamcrest.CoreMatchers.containsString;
-import static org.hamcrest.MatcherAssert.assertThat;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
 
@@ -145,8 +143,8 @@ public void containsWordIgnoreCase_validInputs_correctResult() {
 
     @Test
     public void getDetails_exceptionGiven() {
-        assertThat(StringUtil.getDetails(new FileNotFoundException("file not found")),
-                   containsString("java.io.FileNotFoundException: file not found"));
+        assertTrue(StringUtil.getDetails(new FileNotFoundException("file not found"))
+            .contains("java.io.FileNotFoundException: file not found"));
     }
 
     @Test