tests: Improve stability of Subscribe tests (#6420)

The `Subscribe` tests were flaky, because each test performs some operations (e.g. sends transactions) and waits for messages to appear in subscription with a 100ms timeout. If tests are slow (e.g. compiled in debug mode or a slow machine) then some of them could fail. This change adds an attempt to synchronize the background Env's thread and the test's thread by ensuring that all the scheduled operations are started before the test's thread starts to wait for a websocket message. This is done by limiting I/O threads of the app inside Env to 1 and adding a synchronization barrier after closing the ledger.

src/test/jtx/impl/envconfig.cpp

@@ -87,6 +87,12 @@ secure_gateway_localnet(std::unique_ptr<Config> cfg)
     (*cfg)[PORT_WS].set("secure_gateway", "127.0.0.0/8");
     return cfg;
 }
+std::unique_ptr<Config>
+single_thread_io(std::unique_ptr<Config> cfg)
+{
+    cfg->IO_WORKERS = 1;
+    return cfg;
+}
 
 auto constexpr defaultseed = "shUwVw52ofnCUX5m7kPTKzJdr4HEH";