tests: simplify storage test cleanup

No need to spawn a child 'rm' process when we can do it ourselves.

* tests/virstoragetest.c (testCleanupImages): Use dedicated
helper.
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/virstoragetest.c

@@ -62,8 +62,6 @@ static char *abslink2;
 static void
 testCleanupImages(void)
 {
-    virCommandPtr cmd;
-
     VIR_FREE(qemuimg);
     VIR_FREE(absraw);
     VIR_FREE(canonraw);
@@ -79,9 +77,7 @@ testCleanupImages(void)
         return;
     }
 
-    cmd = virCommandNewArgList("rm", "-rf", datadir, NULL);
-    ignore_value(virCommandRun(cmd, NULL));
-    virCommandFree(cmd);
+    virFileDeleteTree(datadir);
 }
 
 static int