chore(tests): add string test

t/string.c

@@ -0,0 +1,25 @@
+#include "mln_string.h"
+#include <assert.h>
+
+int main(void)
+{
+    mln_string_t text = mln_string("Hello");
+    mln_string_t *slices, *ref, *s = mln_string_dup(&text);
+    assert(s != NULL);
+
+    assert((ref = mln_string_ref(s)) == s);
+
+    assert(!mln_string_strcmp(ref, &text));
+
+    assert((slices = mln_string_slice(ref, "e")) != NULL);
+
+    assert(slices[0].len > 0);
+    assert(!mln_string_const_strcmp(&slices[0], "H"));
+    assert(slices[1].len > 0);
+    assert(!mln_string_const_strcmp(&slices[1], "llo"));
+
+    mln_string_slice_free(slices);
+    mln_string_free(ref);
+    mln_string_free(s);
+    return 0;
+}