[init] Add new command line arg for use only in functional tests

some of the existing command line args are to be only used in
functional tests. ex: addrmantest, fastprune etc.. make a separate
category -test=<option> for these so that code is cleaner and
user's debug-help output is straightforward.

src/common/args.cpp

@@ -682,6 +682,17 @@ std::string HelpMessageOpt(const std::string &option, const std::string &message
            std::string("\n\n");
 }
 
+const std::vector<std::string> TEST_OPTIONS_DOC{
+};
+
+bool HasTestOption(const ArgsManager& args, const std::string& test_option)
+{
+    const auto options = args.GetArgs("-test");
+    return std::any_of(options.begin(), options.end(), [test_option](const auto& option) {
+        return option == test_option;
+    });
+}
+
 fs::path GetDefaultDataDir()
 {
     // Windows: C:\Users\Username\AppData\Roaming\Bitcoin