Implement assert_str and assert_not_null and update tests

v0/test.h

@@ -12,4 +12,25 @@ typedef void (*Test)(void);
  */
 void fail(const char* msg);
 
+/**
+ * Asserts that a pointer is not null.
+ * 
+ * Calls `fail` if the assertion does not hold.
+ * 
+ * @param ptr The pointer to test.
+ * @param msg The message to print if the pointer is null.
+ */
+void assert_not_null(void* ptr, const char* msg);
+
+/**
+ * Asserts that a string has the expected value.
+ * 
+ * Calls `fail` if the assertion does not hold.
+ * 
+ * @param expected The expected value. This is typically a string literal.
+ * @param actual The actual value. This is typically an expression.
+ * @param msg The message to print if these do not match.
+ */
+void assert_str(const char* expected, const char* actual, const char* msg);
+
 #endif