qt, test: Clean settings after tests

A test suite should not leave any artifacts except for those explicitly
expected.

This change is easy to review with `git diff --ignore-all-space`
command.

src/qt/test/test_main.cpp

@@ -85,13 +85,16 @@ int main(int argc, char* argv[])
         setenv("QT_QPA_PLATFORM", "minimal", 0 /* overwrite */);
     #endif
 
-    BitcoinApplication app;
-    app.setOrganizationName(QAPP_ORG_NAME);
-    app.setApplicationName(QAPP_APP_NAME_DEFAULT "-test");
-    app.createNode(*init);
+
+    QCoreApplication::setOrganizationName(QAPP_ORG_NAME);
+    QCoreApplication::setApplicationName(QAPP_APP_NAME_DEFAULT "-test");
 
     int num_test_failures{0};
 
+    {
+        BitcoinApplication app;
+        app.createNode(*init);
+
         AppTests app_tests(app);
         num_test_failures += QTest::qExec(&app_tests);
 
@@ -117,5 +120,10 @@ int main(int argc, char* argv[])
         } else {
             qDebug("\nAll tests passed.\n");
         }
+    }
+
+    QSettings settings;
+    settings.clear();
+
     return num_test_failures;
 }