Remove conflicting REQUIRE definition in unit tests

* undef reference_model def to allow doctest def

Signed-off-by: James Ward <james.ward@arm.com>
Change-Id: I143a7d57344dd3c19248c30ced1aca1d770d47a3
diff --git a/reference_model/test/model_runner_tests.cpp b/reference_model/test/model_runner_tests.cpp
index cc295d9..8304bc7 100644
--- a/reference_model/test/model_runner_tests.cpp
+++ b/reference_model/test/model_runner_tests.cpp
@@ -20,6 +20,9 @@
 #include "model_runner.h"
 #include "general_utils.h"
 
+// Remove conflicting REQUIRE definition between doctest and reference_model
+#undef REQUIRE
+
 #include "doctest.h"
 
 using namespace TosaReference;