While doing a5094338 (encoder: More unexpected EOF handling) I was a bit
disappointed that https://golang.org/cl/37052 was not going to be
accepted and somehow added hack to tests that was doing

	strconv.ErrSyntax -> io.ErrUnexpectedEOF

error conversion if the test name contained "unicode".

Today I was refactoring tests and noticed that it is better we teach loadUnicode
to return io.ErrUnexpectedEOF in the first place, and the next easy way for this
(after fixing strconv.UnquoteChar itself) is to append many "0" to string and
see if strconv.UnquoteChar still returns ErrSyntax.

So do it in our custom unquoteChar wrapper.