Added test covering removal of double negations.

tests/TestPDDLNormalization.cpp

@@ -0,0 +1,23 @@
+#include <gtest/gtest.h>
+
+#include <plasp/pddl/expressions/Not.h>
+#include <plasp/pddl/expressions/Unsupported.h>
+
+using namespace plasp::pddl;
+
+////////////////////////////////////////////////////////////////////////////////////////////////////
+
+TEST(PDDLNormalizationTests, DoubleNegation)
+{
+	auto n1 = std::make_unique<expressions::Not>();
+	auto n2 = std::make_unique<expressions::Not>();
+	auto u = std::make_unique<expressions::Unsupported>();
+	const auto up = u.get();
+
+	n2->setArgument(std::move(u));
+	n1->setArgument(std::move(n2));
+
+	auto normalized = n1->normalize();
+
+	ASSERT_EQ(normalized.get(), up);
+}