Merge #16975: test: Show debug log on unit test failure

fa37e0a68b test: Show debug log on unit test failure (MarcoFalke)

Pull request description:

  Often, it is hard to debug unit test failures without the debug log. Especially when the failure happens remotely (e.g. on a ci system).

  Fix that by printing the log on failure.

ACKs for top commit:
  jamesob:
    ACK fa37e0a68b ([`jamesob/ackr/16975.1.MarcoFalke.test_show_debug_log_on_u`](https://github.com/jamesob/bitcoin/tree/ackr/16975.1.MarcoFalke.test_show_debug_log_on_u))

Tree-SHA512: 2ca4150c4ae3d4ad47e03b5e5e70da2baffec928ddef1fdf53a3ebc061f14aee249205387cb1b12ef6d4eb55711ef0080c0b41d9d18000b5da124ca80299793b

src/qt/test/test_main.cpp

@@ -37,6 +37,8 @@ Q_IMPORT_PLUGIN(QCocoaIntegrationPlugin);
 #endif
 #endif
 
+const std::function<void(const std::string&)> G_TEST_LOG_FUN{};
+
 // This is all you need to run all the tests
 int main(int argc, char *argv[])
 {