chore: Update tests in UnicodeHandling

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Motivation:
refs: #508

@JoshRosen said, in both go/cpp jsonnet has behavior :

    // Unpaired surrogate handling - sjsonnet-specific behavior
    //
    // Note: This is an intentional divergence from go-jsonnet and C++ jsonnet:
    // - go/C++ reject unpaired surrogates in escape sequences at parse time
    // - go-jsonnet's std.char() replaces surrogate codepoints with U+FFFD
    // - sjsonnet preserves unpaired surrogates throughout
    //
    // This permissive behavior is maintained for backwards compatibility.

But in my recent PR, which actually will reject the wrong Unicode.

@JoshRosen @stephenamar-db Should we do this before 1.0.0? I think so, because the wrong Unicode will cause unexpected behavior anyway.

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@JoshRosen the new behavior will fail now

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Error: example1.jsonnet:1:1-9 example1.jsonnet:1:1-9 Truncated unicode surrogate pair escape sequence in string literal.

"\uD800"

in jsonnet.org

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@stephenamar-db I think this should be merged to get other PR's tests green.