removed unused feature in buildslave.scripts.runner.stop()

No code calls stop() with wait flag disabled. Remove wait parameter,
and changed stop() to always wait for the process to finish.
copy StdoutAssertionsMixin class to slave test util package
tests: broke out open() patching code to a mix in class

Created buildslave.test.util.misc.OpenFileMixin class with methods
for patching open() to simulate successful reads and I/O errors.
fixed incorrectly printed message on 'buildslave restart'

Don't print 'no old buildslave process found to stop' when old
slave process was successfully terminated.

Previously stop() returned 'None' when slave was successfully
stoped. To restart() it looked like we failed to stop the slave.

Broke out the code to stop slave into a new stopSlave() utility
function, which will raise an exception if no slave is running.
Use stopSlave() from stop() and restart(), handle the exception
as appropriate for each case.