Run dotty tests with the correct baseDirectory
Run dotty tests with the correct baseDirectory

Before this commit, Build.scala contained:

  baseDirectory in run := baseDirectory.value / "..",

The goal of this line is that when invoking the compiler using `run` or
`dotc`, the working directory is the root of the dotty project, which is
convenient for testing.

However, this line also affects the working directory of
`dotty-compiler/runPartestRunner`, but not of `dotty-compiler/test`.
The result was that `dotty-compiler/test` was broken, and fixing the
paths in the code would break `partest`.

We solve this by using a different baseDirectory when running partest.

dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,dotty-staging/dotty,dotty-staging/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,lampepfl/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,sjrd/dotty,lampepfl/dotty,dotty-staging/dotty,som-snytt/dotty