Ok, I solved the problem.
Material Icons can only use uppercase Latin text. When I used "Testing" for the button caption it replaced it with "T ". This may be a problem if I ever want to use lowercase text along with the icon. I may have to resort to converting the Material Icon to a bitmap and merging it with lowercase text. But this should be infrequent because I believe the Android standard is to use uppercase text on buttons. Even so, I'm still not sure why Material Icons didn't include lowercase text because it takes up only 26 locations in their font.
FontAwesome can display upper or lower case Latin text so "Testing" shows up just fine. I guess that's why they call it "awesome".
Anyways, I thought I'd let you know what the solution is. I'm sure I won't be the only one to encounter the problem. It will save someone a lot of head scratching I'm sure.