Set default signal handlers for SIGINT and SIGHUP.

According to test_misc.py comments, fedora's build
agent has the same problem as ours.  We use the setUp
and tearDown methods to set and restore these signals,
if ignored.
Resolve AttributeError in restore_ignored_signals
use signal_value, resolving TypeError of getattr()
Allow coverage to report even on failing tests

This prevents the obscure "artifact dependencies could
not be resolved" message in TeamCity and is favored by
the more appropriate "such and such tests failed".
exit 0 for for runtests, #test failures dominate
spellfix of 'exit'
Merge pull request #148 from jquast/unset-buildagent-signal-handlers

Unset ignored signal handlers by build agents