Import Dexter to debuginfo-tests

Dexter (Debug Experience Tester) is a test-driver for our debug info
integration tests, reading a set of debug experience expectations and
comparing them with the actual behaviour of a program under a debugger.
More about Dexter can be found in the RFC:

  http://lists.llvm.org/pipermail/llvm-dev/2019-October/135773.html

and the phab review in D68708. Not all the debuginfo tests have been
transformed into Dexter tests, and we look forwards to doing that
incrementally.

This commit mostly aims to flush out buildbots that are running
debuginfo-tests but don't have python 3 installed, possibly
green-dragon and some windows bots.