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

[3.14] gh-127146: Emscripten: Fix test_open_undecodable_uri by setting -sTEXTDECODER=2 (GH-136624) #189743