test: improve wasi stat test

This commit improved the stat test a bit by verifying that
S_ISDIR() works properly. It also adds missing coverage for
__wasi_path_remove_directory().

PR-URL: #31413
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/wasi/c/stat.c

@@ -49,5 +49,15 @@ int main(void) {
   ret = stat(PATH, &st);
   assert(ret == -1);
 
+  ret = stat(OUTPUT_DIR, &st);
+  assert(ret == 0);
+  assert(S_ISDIR(st.st_mode));
+
+  ret = rmdir(OUTPUT_DIR);
+  assert(ret == 0);
+
+  ret = stat(OUTPUT_DIR, &st);
+  assert(ret == -1);
+
   return 0;
 }