Update mantle-framework/testkit requirement from ^0.5 to ^0.6

[dependabot[bot]]

Updates the requirements on mantle-framework/testkit to permit the latest version.

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[srtfisher]

@dependabot rebase

[dependabot[bot]]

Looks like this PR is already up-to-date with main! If you'd still like to recreate it from scratch, overwriting any edits, you can request @dependabot recreate.

[srtfisher]

@dependabot recreate