added test to make sure hexStrLen function behaves as corrected

test/conversionOfNumbers/test_conversionOfNumbers.c

@@ -153,3 +153,15 @@ void test_hexStringLengthOfInput256is3(){
     //assert
     TEST_ASSERT_EQUAL_INT(expected, result);
 }
+
+void test_hexStringLengthOfInput255is2(){
+    //arrange
+    int result;
+    int expected = 2;
+
+    //act
+    result = hexStrLen(255);
+
+    //assert
+    TEST_ASSERT_EQUAL_INT(expected, result);
+}