Move the TestPaths structure from libtest to compiletest.

This structure doesn't seem to be used by libtest itself. It is used by
compiletest, but never passed on to anything externally. This makes it
easier to get the testing framework to work for CloudABI crossbuilds, as
CloudABI currently lacks PathBuf, which is used by TestPaths.

src/libtest/lib.rs

@@ -203,13 +203,6 @@ pub struct TestDesc {
     pub allow_fail: bool,
 }
 
-#[derive(Clone)]
-pub struct TestPaths {
-    pub file: PathBuf,         // e.g., compile-test/foo/bar/baz.rs
-    pub base: PathBuf,         // e.g., compile-test, auxiliary
-    pub relative_dir: PathBuf, // e.g., foo/bar
-}
-
 #[derive(Debug)]
 pub struct TestDescAndFn {
     pub desc: TestDesc,