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.

Author: smarter

project/Build.scala

@@ -179,8 +179,10 @@ object DottyBuild extends Build {
         ) map { case (k, v) => (k, v.getAbsolutePath) }
       },
 
-      // Set run baseDir to be root of project, makes dotc saner
-      baseDirectory in run := baseDirectory.value / "..",
+      // For convenience, change the baseDirectory when running the compiler
+      baseDirectory in (Compile, run) := baseDirectory.value / "..",
+      // .. but not when running partest
+      baseDirectory in (Test, run) := baseDirectory.value,
 
       repl := Def.inputTaskDyn {
         val args: Seq[String] = spaceDelimited("<arg>").parsed