Fix Unicode output in `cabal-testsuite`

Fix Unicode output in `cabal-testsuite`

`System.Process.createPipe` calls (through many intermediaries)
`GHC.IO.Handle.FD.fdToHandle`, whose documentation says:

> Makes a binary Handle. This is for historical reasons; it should
> probably be a text Handle with the default encoding and newline
> translation instead.

The documentation for `System.IO.hSetBinaryMode` says:

> This has the same effect as calling `hSetEncoding` with `char8`, together
> with `hSetNewlineMode` with `noNewlineTranslation`.

But this is a lie, and Unicode written to or read from binary handles is
always encoded or decoded as Latin-1, which is always the wrong choice.

Therefore, we explicitly set the output to UTF-8 to keep it consistent
between platforms and correct on all modern computers.

See: https://gitlab.haskell.org/ghc/ghc/-/issues/25307