This is obviously going to be something particular to everyone's machine, but I find when I try to run wpt on my machine with a non-opt build that I run into the 10s timeout fairly frequently. Any chance of increasing it? (I tend to work with non-opt builds for ease of debugging.)
In theory the mozinfo.json file in your objdir should be read to determine that the build is a debug build and then the timeout should be increased by a factor of (typically) 3. It's possible that this is broken somehow; try passing --debug-build and see if that helps. Alternatively if 30/180s is still not a long enough timeout try passing --timeout-multiplier N for some N > 3.