Retry debug test failures that hit a USB error.

Each test will be attempted 5 times, if necessary, when the output
includes "Timed out handling USB events in mpsse_flush" (from
https://github.com/riscv/riscv-openocd/pull/301). If that happens,
output will look something like this:
```
[TooManyHwbp] Starting attempt #0 > logs/20181005-125625-spike32-TooManyHwbp.log
[TooManyHwbp] usb-error in 4.65s
[TooManyHwbp] Starting attempt #1 > logs/20181005-125630-spike32-TooManyHwbp.log
[TooManyHwbp] pass in 4.43s
```