Split bugreport() into its own file and added unit tests.

bugreport() will be soon refactored to track progress, which will
require more comprehensive unit tests.

As such, it's better to move it to its own files, which in turn also
requires moving send_shell_command() and usage() to commandline.h.

Fixes: 30100363
Bug: 30268737

Change-Id: I3cdf114a0b5547293320042ff0749a60886440b0