Fix failing mozprocess tests on windows

- Adjust returncode logic in processhandler.py to return correct error code on
unix (instead of always returning 0). Now if a process is killed it returns -9
(-SIGKILL).

- Delete broken unix wait() implementation (it did not parse out the process
returncode correctly). Pass through to subprocess.Popen.wait().

- Call sleep() between killing a process and reading its return code, otherwise
return code is sometimes/always not set in time by the OS.

- test_process_kill and test_process_timeout check for platform-specific return
code.

- check_for_process works on win32 (search for only filename in tasklist
output, instead of full path).

- Kill process at end of test_process_waittimeout. Was causing 'access denied'
error on windows when deleting binary in tearDownClass
Adjust process run times in mozprocess tests to be significantly shorter