Minor updates to docs on how to run tests (#8666)

The test runner isn't present in the `[all]` set of extras, so the
previous instructions did not work without also installing `[test]`.

Note that this does not include the `[lint]` extras, since those do not
install on all supported Python versions (specifically, isort 5.x
requires Python 3.6, while we still support 3.5). Instructions for that
are included in our pull request template, so we should be fine there.

I've also dropped the `--no-use-pep517` arg to `pip install` since it
seems to have been added to address a temporary regression in pip 19.1
which was fixed in pip 19.1.1 the following month.

Lastly, updated the example output of the test suite to set more
realistic expectations around run time.

Signed-off-by: Dan Callahan <danc@element.io>