This removes the majority of `../../../../../../test_util` relative
imports from the codebase, allowing us to move this code more easily in
the future.
This commit marks the `Deno.Buffer` / `Deno.readAll` /
`Deno.readAllSync` / `Deno.writeAll` / `Deno.writeAllSync` utils as
deprecated, and schedules them for removal in Deno 2.0. These
utilities are implemented in pure JS, so should not be part of the
Deno namespace.
These utilities are now available in std/io/buffer and std/io/util:
https://github.com/denoland/deno_std/pull/808.
This additionallty removes all internal dependance on Deno.Buffer.
