ghcup metadata gives incorrect dlSubdir of testsuite artifact

As noted in https://github.com/haskell/ghcup-metadata/pull/186#discussion_r1524064965, the dlSubdir of the testsuite artifact in the generated ghcup metadata is incorrect. It is listed as dlSubdir: ghc-$ver whereas it should be dlSubdir: ghc-$ver/testsuite.

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