Move `cmd/buildah/passwd.go` to `tests/`

[mtrmac]

cmd/buildah/passwd.go adds a hidden, undocumented, buildah passwd command.

This was added in #3894 and it is used to set up a test environment.

We don’t need to ship this subcommand (and the Bcrypt implementation, which seems to have no other users in the binary) to users; removing that would save a few bytes per user, and simplify our analysis (e.g. about using FIPS-unapproved algorithms).

[github-actions]

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