Add a guide for the Python testing framework.

[wmarshall484]

No description provided.

[ddebrunner]

As a reference the API doc is here (with an example):

http://ibmstreams.github.io/streamsx.topology/doc/releases/1.6/pythondoc/streamsx.topology.tester.html

[jakrmpotic]

@ddebrunner some copy edits on the ref docs:

Add missing commas:
Thus, currently testing
If STREAMS_INSTALL is not set, then
a service name is not defined, then
evaluates to False, then
If exact is True, then condition becomes
If exact is False, then the condition becomes

Fix word stutter:
Test that that a stream
running until the all the conditions

Change “an” to “a”:
contains an number of tuples

Change “on stream” to “on a stream” (global comment)
are seen on stream

Typos:
Use Assertion (?) in two places: AssetionError
Use Streams: The Steams instance
Use Set up: Setup a unittest.TestCase …

Canceled/Cancelled -> Use one spelling consistently. We use diff spellings of “canceled” and “cancelled” in one para.

Remove “an” to make sentence grammatically correct (or rewrite as appropriate): … allows an additional conditions that are implementations of Condition.