Do not insert SPDX identifier if already present in SyntaxTest

test/libsolidity/SyntaxTest.h

@@ -50,6 +50,9 @@ public:
 	TestResult run(std::ostream& _stream, std::string const& _linePrefix = "", bool _formatted = false) override;
 
 protected:
+	/// Returns @param _sourceCode prefixed with the version pragma and the SPDX license identifier.
+	static std::string addPreamble(std::string const& _sourceCode);
+
 	void setupCompiler();
 	void parseAndAnalyze() override;
 	virtual void filterObtainedErrors();