Unset ignored signal handlers by build agents

**major**
- TeamCity's build agent starts with SIGINT and SIGHUP ignored, which python honors, causing some tests to fail around terminate().
- These are now saved, reset to default, and restored in setUp and tearDown of PexpectTestCase.
- There is also some code in tests/test_misc.py about "fedora's build servers" that this replaces.

**minor**
- allow coverage reporting to complete even if tests fail -- this removes the obscure "Failed to resolve artifact dependency" build error.

This allows Linux build to pass on TeamCity, reaching parity with the travis-ci builds:

https://teamcity-master.pexpect.org/viewLog.html?buildId=763&tab=buildResultsDiv&buildTypeId=Pexpect_LinuxBuild

We have EOF errors remaining for OSX and SunOS.
exit