[mlir][test] Add missing LIT config for `mlir-cpu-config` + emulator

Similarly to when using `lli`, make sure that when using
`mlir-cpu-runner` with an emulator, a full path to `mlir-cpu-runner` is
used. Otherwise `mlir-cpu-runner` won't be found and you will get the
following error:
```
Error while loading mlir-cpu-runner: No such file or directory
```

This patch should fix:
  * https://lab.llvm.org/buildbot/#/builders/179
The breakage was originally introduced in
https://reviews.llvm.org/D155405.

Differential Revision: https://reviews.llvm.org/D155920