Backport of Fix linter issues in policy.go & acl.go into release/1.10.x

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #16366 to be assessed for backporting due to the inclusion of the label backport/1.10.x.

The below text is copied from the body of the original PR.

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Fixing a few small issues I noticed while looking into policy parameter constraints:

• nil was passed to context in tests; replaced it with context.Background()
• t.Fatal was being used inside of internal go-routines, which would not fail the test; changed the test to push errors into a channel and check them at the end
• use copy and append instead of loops to copy/append slices

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Overview of commits

• 1e515ca08cc390b88e4692d9419cfd818335bbbd