While something like word-wrap: break-word;
could momentarily fix your problem, it’s not anywhere near an ideal solution.
When using the visual editor, do you have it set to Visual or HTML? When live editing the code of your site and adding in some filler text in place of the Testing, I see that it is breaking properly. As alchymyth has pointed out above, you are using in your text. As the name implies, it is non-breaking, meaning it will continue to go until a break is found.
You mentioned that it happens with each theme that you tried, so my guess is just in how you copied and pasted the testing in. I see on other posts that other text properly breaks.