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: https://github.com/nodejs/node/pull/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>