tests/extmod: Add (failing) test for VfsPosix in readonly mode.

I noticed that operations such as unlink could be performed on a nominally
read-only VfsPosix.

Signed-off-by: Jeff Epler <jepler@gmail.com>

tests/extmod/vfs_posix_readonly.py

@@ -0,0 +1,99 @@
+# Test for VfsPosix
+
+try:
+    import gc, os, vfs, errno
+
+    vfs.VfsPosix
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+# We need a directory for testing that doesn't already exist.
+# Skip the test if it does exist.
+temp_dir = "vfs_posix_readonly_test_dir"
+try:
+    os.stat(temp_dir)
+    raise SystemExit("Target directory {} exists".format(temp_dir))
+except OSError:
+    pass
+
+# mkdir (skip test if whole filesystem is readonly)
+try:
+    os.mkdir(temp_dir)
+except OSError as e:
+    if e.errno == errno.EROFS:
+        print("SKIP")
+        raise SystemExit
+
+fs_factory = lambda: vfs.VfsPosix(temp_dir)
+
+# mount
+fs = fs_factory()
+vfs.mount(fs, "/vfs")
+
+with open("/vfs/file", "w") as f:
+    f.write("content")
+
+# test reading works
+with open("/vfs/file") as f:
+    print("file:", f.read())
+
+os.mkdir("/vfs/emptydir")
+
+# umount
+vfs.umount("/vfs")
+
+# mount read-only
+fs = fs_factory()
+vfs.mount(fs, "/vfs", readonly=True)
+
+# test reading works
+with open("/vfs/file") as f:
+    print("file 2:", f.read())
+
+# test writing fails
+try:
+    with open("/vfs/test_write", "w"):
+        pass
+    print("opened")
+except OSError as er:
+    print(repr(er))
+
+# test removing fails
+try:
+    os.unlink("/vfs/file")
+    print("unlinked")
+except OSError as er:
+    print(repr(er))
+
+# test renaming fails
+try:
+    os.rename("/vfs/file2", "/vfs/renamed")
+    print("renamed")
+except OSError as er:
+    print(repr(er))
+
+# test removing directory fails
+try:
+    os.rmdir("/vfs/emptydir")
+    print("rmdir'd")
+except OSError as er:
+    print(repr(er))
+
+# test creating directory fails
+try:
+    os.mkdir("/vfs/emptydir2")
+    print("mkdir'd")
+except OSError as er:
+    print(repr(er))
+
+# umount
+vfs.umount("/vfs")
+
+fs = fs_factory()
+vfs.mount(fs, "/vfs")
+
+os.rmdir("/vfs/emptydir")
+os.unlink("/vfs/file")
+
+os.rmdir(temp_dir)

tests/extmod/vfs_posix_readonly.py.exp

@@ -0,0 +1,7 @@
+file: content
+file 2: content
+OSError(30,)
+OSError(30,)
+OSError(30,)
+OSError(30,)
+OSError(30,)