fixup! test: Fix failing exception check in new thread busy test

Author: ryanofsky

test/mp/test/test.cpp

@@ -10,7 +10,6 @@
 #include <capnp/rpc.h>
 #include <condition_variable>
 #include <cstring>
-#include <exception>
 #include <functional>
 #include <future>
 #include <kj/async.h>
@@ -23,14 +22,15 @@
 #include <kj/test.h>
 #include <memory>
 #include <mp/proxy.h>
-#include "mp/proxy.capnp.h"
+#include <mp/proxy.capnp.h>
 #include <mp/proxy-io.h>
-#include "mp/util.h"
+#include <mp/util.h>
 #include <optional>
 #include <set>
 #include <stdexcept>
 #include <string>
 #include <string_view>
+#include <system_error>
 #include <thread>
 #include <utility>
 #include <vector>