gui tests: Limit life-time of dummy testing setup

Its only purpose is to create a directory. So instead of keeping it
alive, use it only to get the path of the directory and then create it
explicitly.

src/qt/test/apptests.cpp

@@ -62,9 +62,10 @@ void AppTests::appTests()
     }
 #endif
 
+    fs::create_directories([] {
         BasicTestingSetup test{CBaseChainParams::REGTEST}; // Create a temp data directory to backup the gui settings to
-    ECC_Stop(); // Already started by the common test setup, so stop it to avoid interference
+        return GetDataDir() / "blocks";
-    LogInstance().DisconnectTestLogger();
+    }());
 
     m_app.parameterSetup();
     m_app.createOptionsModel(true /* reset settings */);
@@ -80,6 +81,7 @@ void AppTests::appTests()
     m_app.exec();
 
     // Reset global state to avoid interfering with later tests.
+    LogInstance().DisconnectTestLogger();
     AbortShutdown();
     UnloadBlockIndex();
     WITH_LOCK(::cs_main, g_chainman.Reset());