Do not insert SPDX identifier if already present in SyntaxTest

test/libsolidity/syntaxTests/license/license_missing.sol

@@ -1 +1,2 @@
+// This test is actually useless, as the test suite adds the automatic preamble.
 contract C {}

test/libsolidity/syntaxTests/license/license_unicode.sol

@@ -1,3 +1,4 @@
-// This is just ignored:
 // SPDX-License-Identifier: ⡉⡊⡋⡌⡍⡎⡏⡐⡑⡒
 contract C {}
+// ----
+// Warning 1878: SPDX license identifier not provided in source file. Before publishing, consider adding a comment containing "SPDX-License-Identifier: <SPDX-License>" to each source file. Use "SPDX-License-Identifier: UNLICENSED" for non-open-source code. Please see https://spdx.org for more information.