redpanda/admin: remove throw statement in start_handler

This commit removes the `throw` statement in
`admin_server::self_test_start_handler()` and replaces it instead
with a `push_back()` to `self_test_request::unknown_checks`.

Eventually, these unknown checks will have a result displayed
when `rpk cluster self-test status` is invoked.

For the self-test, any unrecognized tests will be appended to
`start_test_request::unknown_checks`, so a future result from
`rpk cluster self-test status` will return a message indicating
an unknown test was skipped.

src/v/redpanda/admin/server.cc

@@ -2819,9 +2819,13 @@ admin_server::self_test_start_handler(std::unique_ptr<ss::http::request> req) {
                 } else if (test_type == "cloud") {
                     r.ctos.push_back(cluster::cloudcheck_opts::from_json(obj));
                 } else {
-                    throw ss::httpd::bad_param_exception(
-                      "Unknown self_test 'type', valid options are 'disk', "
-                      "'network', or 'cloud'.");
+                    rapidjson::StringBuffer buffer;
+                    rapidjson::Writer<rapidjson::StringBuffer> writer(buffer);
+                    element.Accept(writer);
+                    r.unknown_checks.push_back(cluster::unknown_check{
+                      .test_type = test_type,
+                      .test_json = ss::sstring{
+                        buffer.GetString(), buffer.GetSize()}});
                 }
             }
         } else {