Update compiletest target path

Without this change it accidentally used
tests/target/compiletest-{deps,results} instead of the workspace target dir

@eddyb pointed this out.

Author: LegNeato

tests/compiletests/src/main.rs

@@ -60,7 +60,7 @@ fn main() {
     let opt = Opt::parse();
 
     let tests_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
-    let workspace_root = tests_dir.parent().unwrap();
+    let workspace_root = tests_dir.parent().unwrap().parent().unwrap();
     let original_target_dir = workspace_root.join("target");
     let deps_target_dir = original_target_dir.join("compiletest-deps");
     let compiletest_build_dir = original_target_dir.join("compiletest-results");