Use actual negative zero in tests

[makew0rld]

Description

This actually fixes the negative zero issue in the tests rather than just ignoring it. For the origin of this see here.

I caught this by running the actual staticcheck binary, which seems to be higher quality than golangci-lint's version of staticcheck rules.

❯ staticcheck ./...
decode_test.go:2088:31: in Go, the floating-point literal '-0.0' is the same as '0.0', it does not produce a negative zero (SA4026)
decode_test.go:2089:37: in Go, the floating-point literal '-0.0' is the same as '0.0', it does not produce a negative zero (SA4026)
decode_test.go:2089:52: in Go, the floating-point literal '-0.0' is the same as '0.0', it does not produce a negative zero (SA4026)
decode_test.go:5250:4: field x is unused (U1000)
decode_test.go:5254:4: field y is unused (U1000)

❯ staticcheck -explain SA4026
Go constants cannot express negative zero

In IEEE 754 floating point math, zero has a sign and can be positive
or negative. This can be useful in certain numerical code.

Go constants, however, cannot express negative zero. This means that
the literals -0.0 and 0.0 have the same ideal value (zero) and
will both represent positive zero at runtime.

To explicitly and reliably create a negative zero, you can use the
math.Copysign function: math.Copysign(0, -1).

Available since
    2021.1

Online documentation
    https://staticcheck.dev/docs/checks#SA4026

PR Was Proposed and Welcomed in Currently Open Issue

• This PR was proposed and welcomed by maintainer(s) in issue #___
• Closes or Updates Issue #___

Checklist (for code PR only, ignore for docs PR)

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[fxamacker]

Thank you!