singleflight: Fix TestPanicErrorUnwrap

[eikemeier]

TestPanicErrorUnwrap does not test what it seems to test

• "wrappedErrorType" is always false (should be true in the second case)
• The values are already wrapped in the private error (they shouldn't)
• "errors.Is(err, new(T))" is undefined for zero-sized "T"s (and false for non-zero-sized).
• "tc := tc" is not needed since Go 1.22

Fixes:

• Use a (local) sentinel value instead of a type, as in TestDoErr.
• Replace all "new(errValue)" by direct references to the unique sentinel "errValue".
• Replace diagnostic, since the error type we get is internal, so we can't directly expect that - it should only wrap our sentinel.

See my blog post at https://blog.fillmore-labs.com/posts/zerosized-2/ for a extended discussion.

Found by zerolint https://github.com/fillmore-labs/zerolint

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Message from Gopher Robot:

Patch Set 1:

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Message from Jorropo:

Patch Set 1: Code-Review+2 Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-07-04T23:23:35Z","revision":"18045c4ea6c2ee676fda25dde409a1621812b6c2"}

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Patch Set 1: -Commit-Queue

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This CL has passed the run

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Sean Liao:

Patch Set 2: Code-Review+1

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Message from Sean Liao:

Patch Set 2: -Code-Review

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