[3.14] gh-127146: Emscripten: Fix test_open_undecodable_uri by setting `-sTEXTDECODER=2` (GH-136624) (#136631)

Removes the JS text decoder fallback and gets rid of the bugs due to the differences
in behavior on invalid utf8 strings. See https://github.com/emscripten-core/emscripten/issues/24690.
(cherry picked from commit 283b05052338dd735cd4927011afc3735d9c6c7c)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>