u_boot_pylib: Avoid concurrent execution of only one test

There is no point in spinning up multiple processes if there is only one
test to execute. Add a check for this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/u_boot_pylib/test_util.py b/tools/u_boot_pylib/test_util.py
index d46c3df..fc441a7 100644
--- a/tools/u_boot_pylib/test_util.py
+++ b/tools/u_boot_pylib/test_util.py
@@ -191,7 +191,7 @@
         resultclass=FullTextTestResult,
     )
 
-    if use_concurrent and processes != 1:
+    if use_concurrent and processes != 1 and not test_name:
         suite = ConcurrentTestSuite(suite,
                 fork_for_tests(processes or multiprocessing.cpu_count()))