Wayne. Its not a head gaskit problem from what you said. If you want to test anything look for water in the oil shown by white deposits(mayo, similar to that sometimes normally found on the inside of the oil filler cap/neck). Do this by pulling the oil dip stick out and visually check what the oil looks like. Anything from Gold(new oil) to black(old oil)in colour is fine.
If it has any white/grey/caramel colour in it, is thin and milkey, suspect Head Gaskit.
You say you worried about cost yet your waisting money on a pressure test. The guy who told you the HG had failed was talking 'dangle berries'. He did not do a pressure test, from your description re "turning yellow", this is a sniffer test(for exhaust fumes) and is totally unreliable.
Have been thinking this from the start of your thread and agree with what everyone else is basically saying, that is your HG is fine. You would have other more serious issues if it had failed. Save your money for a new Heater by pass valve. Hth
Above sounds a bit shouty, not ment to be, just factual and to the best of my knowledge.