Improve mockRunLogs

[john-science]

1. Right now, to use mockRunLogs.py in unit testing, you have to access a private member:

armi/armi/tests/test_runLog.py

Lines 197 to 199 in f0d27e7

	with mockRunLogs.BufferLog() as mock:
	# we should start with a clean slate
	self.assertEqual("", mock._outputStream)

Well, that seems silly. Seems to me that attribute should be public. Easy change, though it means a small PR into one downstream repo, that I can see.

2. It is possible this tool could be more discoverable. It currently exists at armi/tests/mockRunLogs.py. Should this be in or near armi/runLogs.py?

[john-science]

It turns out we already have a helper method for this .getStdoutValues(). People just aren't using it.

So I have opened a ticket to fix that.