This change aims to replace all relative import specifiers targeted at
`tests/util/std` with mapped ones (using a `deno.json` file). Towards
updating the `std` git submodule.
Note: tests are not the only part of the codebase that uses `std`. Other
parts, like `tools/`, do too. So, it could be argued that this is a
little misleading. Either way, I'm doing this as discussed with
@mmastrac.
