compiletest: split rmake executable from scratch dir

When implementing support for rmake.rs, I copied over the `$TMPDIR`
directory logic from the legacy Makefile setup. In doing so, I also
compiled recipe `rmake.rs` into executables which unfortunately are
placed into `$TMPDIR` as well.

This causes a problem on Windows where:

- The `rmake.exe` executable is placed in `$TMPDIR`.
- We run the `rmake.exe` as a process.
- The process uses `rmake.exe` inside `$TMPDIR`.
- Windows prevents the .exe file from being deleted when the process
  is still alive.
- The recipe test code tries to `remove_dir_all($TMPDIR)`, which fails
  with access denied because `rmake.exe` is still being used.

We fix this by separating the recipe executable and the sratch
directory:

```
base_dir/
    rmake.exe
    scratch/
```

We construct a base directory, unique to each run-make test, under
which we place rmake.exe alongside a `scratch/` directory. This
`scratch/` directory is what is passed to rmake.rs tests as `$TMPDIR`,
so now `remove_dir_all($TMPDIR)` has a chance to succeed because
it no longer contains `rmake.exe`.

Oops. This was a fun one to try figure out.

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