The way to clean the socket is the get a dental probe that has a flat contour- The ones they stick between your teeth. You will need this for a little scraping.
The other item is called "Rail Zip". It is a model railroad track cleaner that works well in restoring contact area conductivity.
Either dribble a drop into each socket hole, or try to get good at carrying a drop on the dental probe. Just place a drop in each contact hole, and first try this to see if it is adequate to make the tubes register on the tester. If not, you need to use the probe to break out the non conductive material, and try to work the Rail Zip into the corroded area, then test the tubes again.
:Hi, I have a old tube tester, but all the sockets are very durty. On the contacts, I can see a lot of white-gray deposit. It look like salt! But it's not. This deposit make bad contacts, so all my tubes failed the test! What should I use? Contact cleaner or do I have to scrub? With what? "Q-tips" are to big for the little hole! Thanks for your help.