[Dexter] Avoid infinite loop in dbgeng driver

This method of the dbgeng debugger driver used to just "pass", as setting
dbgeng free running still leads to numerous errors. This wasn't a problem
in the past because, as it turns out, nothing called the go method.
However, a recent refactor uses it.

Rather than launch dbgeng free running, instead have it single step one
step forwards. This is slow, but it makes progress, where previously we
weren't.

Differential Revision: https://reviews.llvm.org/D91737