tests/.gitignore: ignore *.o under tests/src/test_helpers

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/tests/.gitignore b/tests/.gitignore
index b85d66a..40ad061 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -18,6 +18,7 @@
 include/test/instrument_record_status.h
 
 src/*.o
+src/test_helpers/*.o
 src/drivers/*.o
 src/libmbed*