debug: add DeleteBreakpoints function

DeleteBreakpoints accepts a slice of addresses, and removes any breakpoints
set at those addresses.

Adds a test where some breakpoints are set and deleted in the program
being debugged, and we check the program stops at the expected place.

Change-Id: I4ea4718e21e51587e48bd2eb57033ca774004e1a
Reviewed-on: https://go-review.googlesource.com/10079
Reviewed-by: Rob Pike <r@golang.org>