docs: clearly deprecate Harness in the testing how-tos

[tonyandrewmeyer]

Small adjustments to the how-to guides on testing to make it clear that Harness is deprecated:

• Tweak the general guide so that it's still basically saying the same thing, but refers to Scenario rather than Harness.
• Adjust example tests to use ops.testing Scenario classes rather than Harness.
• Make sure it's clear that ops[testing] should be installed to write unit tests.
• Use Scenario from ops.testing rather than from scenario.

Note that this does not substantially change the testing documentation: there's a roadmap item to do that later in 25.04.

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[tonyandrewmeyer]

@tmihoc if you have time, feel free to review this, but no problem if not :)

[benhoyt]

Nice, thanks. Approving with some suggested tweaks.

I also noticed that there's a broken link on this page: https://ops.readthedocs.io/en/latest/explanation/testing.html

ops.testing.Harness and/or {ref}``ops-scenario

Can we fix that while we're here?

[dwilding]

Looks great, thanks Tony! I've added a couple of comments about the framework name