Add some missing EXTENSIONS sections to misc tests

Add identify filter for UTF-16, UTF-16LE, UTF-16BE

There was one faulty test in the suite which only passed before because UTF-16 had no
identify filter. After this was fixed, it exposed

Add identify filter for UTF-16, UTF-16LE, UTF-16BE

There was one faulty test in the suite which only passed before because UTF-16 had no
identify filter. After this was fixed, it exposed the problem with the test.

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Return false from failed mb_convert_variables()

If we fail to detect the encoding return false, just like
mb_convert_encoding() does, and the implementation here clearly
intended. Pr

Return false from failed mb_convert_variables()

If we fail to detect the encoding return false, just like
mb_convert_encoding() does, and the implementation here clearly
intended. Previously the "pass" pseudo-incoding was returned.

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Fix assertion failure when failing to detect encoding

Looks like prior to 7.3 this just passed the original string
through. Since 7.3 it returns false. Let's stick with that
behavior.