SyntaxTest: Default-initialize boolean fields without showing the value

- These get re-initialized in constructor anyway. The only purpose if initializing here is our convention to always initialize primitive types at declaration time. We don't want to have to repeat the defaults though.

test/libsolidity/SyntaxTest.h

@@ -48,7 +48,7 @@ protected:
 	void parseAndAnalyze() override;
 	virtual void filterObtainedErrors();
 
-	bool m_optimiseYul = true;
+	bool m_optimiseYul{};
 };
 
 }