test(pack): make various small cleanups

Problem: `vim.pack` tests might benefit from a cleanup

Solution: Do the cleanup:
  - Use `t.retry` instead of full `vim.uv.sleep` to make tests quicker.
  - Use `pack_assert_content` helper for a common task of checking the
    content of a special Lua file inside an installed plugin. This is
    used as a proxy of installed plugin's state on the disk.
  - Use `([[...]]):format()` approach (instead of array of strings) in
    one more place. Should improve readability.

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