In Simpletest example description, use the Testing Framework URL in docs

[bugfolder]

Currently the module description for Simpletest Example says:

An example of simpletest tests to accompany the tutorial at https://api.backdropcms.org/node/41659.

We should use the current docs.b.org URL for that node, which is:

https://docs.backdropcms.org/documentation/testing-framework

Also, it's not really a "tutorial", so better to say:

An example of simpletest tests to accompany the documentation at https://docs.backdropcms.org/documentation/testing-framework.

[avpaderno]

The same link and similar description is used in _simpletest_example_explanation() too. I corrected that in a follow-up.