AK: Add an extremely primitive unit test for String.

This builds for the host system rather than for Serenity.
We need to improve on it a *lot*, but at least it's a place to start.

AK/Tests/TestString.cpp

@@ -0,0 +1,19 @@
+#include <AK/AKString.h>
+
+int main()
+{
+    ASSERT(String().is_null());
+    ASSERT(String().is_empty());
+
+    ASSERT(!String("").is_null());
+    ASSERT(String("").is_empty());
+
+    String test_string = "ABCDEF";
+    ASSERT(!test_string.is_empty());
+    ASSERT(!test_string.is_null());
+    ASSERT(test_string.length() == 6);
+    ASSERT(test_string.length() == strlen(test_string.characters()));
+    ASSERT(!strcmp(test_string.characters(), "ABCDEF"));
+
+    return 0;
+}