Fix deceptively-simple pattern in pure-Lua ustring

The pure-Lua ustring pattern matching functions short-circuit to the
much faster string library when the pattern would match the same against
the raw bytes.

A pattern like "[^a-z]" can match a partial UTF-8 character when applied
bytewise, and so must be detected as unsafe.

Let's also directly test the pure-Lua module, instead of me having to
comment out lines in Scribunto_LuaUstringLibrary::register() whenever I
want to test them.

Change-Id: I91ed3374aadfea379b9db2e13b4248ab20df509e