test: Add concurrencytest to the requirements

This allows tests to run in parallel and speeds up some tests markedly,
particularly with binman. Add it to the requirements.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/py/requirements.txt b/test/py/requirements.txt
index fae8b59..e241780 100644
--- a/test/py/requirements.txt
+++ b/test/py/requirements.txt
@@ -1,5 +1,6 @@
 atomicwrites==1.4.1
 attrs==19.3.0
+concurrencytest==0.1.2
 coverage==4.5.4
 extras==1.0.0
 filelock==3.0.12