LanguageClient: Hide plugin class definition in cpp

As part of that, move test object creation closer to tested code.

Change-Id: I60f1e431fbdd4ebc2a01ab6ed0c779c288c81b06
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/languageclient/snippet.h

@@ -11,4 +11,8 @@ namespace LanguageClient {
 
 LANGUAGECLIENT_EXPORT TextEditor::SnippetParseResult parseSnippet(const QString &snippet);
 
+#ifdef WITH_TESTS
+QObject *createSnippetParsingTest();
+#endif
+
 } // namespace LanguageClient