Better test for the predefined "any" type

[bobg]

This PR checks that the use of any as a type name actually refers to the predefined type introduced in Go 1.18 and doesn't simply denote a (user-defined) empty interface.

[github-actions]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR does not require a change in your module’s version number.
(You might still consider bumping the patchlevel anyway.)

[coveralls]

Pull Request Test Coverage Report for Build 8039285660

Details

• 0 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 61.794%

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Totals
Change from base Build 7817575698:	0.0%
Covered Lines:	820
Relevant Lines:	1327

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💛 - Coveralls