bisync: rename extended_char_paths test

The .lck file filename length needs to be less than 255 bytes (not symbols) on
linux, and it was still too long on this test, because of the
subdir=測試_Русский_{spc}_{spc}_ě_áñ
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f