Share script fixtures across tests

[chrahunt]

We now have a script_factory pytest fixture that we can use to create reusable script objects. These can be used across multiple tests.

This helps because each usage of the plain script fixture requires:

1. copying the whole template virtual environment
2. (usually) copying test data (tests.lib.TestData via the data fixture)
3. some setup/installation that is usually common across multiple tests

I just picked two example functional test files to start - I think there are a lot more opportunities to improve our test speed here.

Makes progress on #4497.

[xavfernandez]

Nice idea 👍
We might want to add a readonly attribute to those shared script to prevent pip install/uninstall calls ?

[chrahunt]

I expect the usage of an individual fixture to be small enough that its intended use should be clear. If they start to get too big then it should be easy enough to add a whitelist on each instance as appropriate.

[xavfernandez]

Did you measure the speedup for tests/functional/test_completion.py & tests/functional/test_list.py ?

[chrahunt]

On my machine™, with times averaged over 5 runs, it looks like:

file	cores	master (s)	new (s)
completion	1	13.4762	8.7662
	2	7.8660	6.6554
list	1	51.3596	39.8602
	2	26.8778	22.6406

Progress on pytest-dev/pytest-xdist#18 would probably improve this some (especially if the files are executed at the same time).

[chrahunt]

Thanks for reviewing @xavfernandez!