Fix running multiple targets.

The aux dir, which previously had the `stage_id` embedded in it, was picking up remnants from previous runs.

src/tools/compiletest/src/runtest.rs

@@ -1476,12 +1476,13 @@ impl<'test> TestCx<'test> {
     }
 
     fn compose_and_run_compiler(&self, mut rustc: Command, input: Option<String>) -> ProcRes {
+        let aux_dir = self.aux_output_dir_name();
+
         if !self.props.aux_builds.is_empty() {
-            create_dir_all(&self.aux_output_dir_name()).unwrap();
+            let _ = fs::remove_dir_all(&aux_dir);
+            create_dir_all(&aux_dir).unwrap();
         }
 
-        let aux_dir = self.aux_output_dir_name();
-
         for rel_ab in &self.props.aux_builds {
             let aux_testpaths = self.compute_aux_test_paths(rel_ab);
             let aux_props =