Allow running tests against the installed version

[wRAR]

Fixes #4914 by using get_testenv() only for python3 -m scrapy.cmdline and finding files under tests via different mechanisms.

For some reason running these as-installed tests in the Debian autopkgtest chroot always fails on tests/test_crawl.py::CrawlTestCase::test_start_requests_laziness with "All start requests consumed before any download happened". I've looked at the test and the spider it runs and I don't understand if it just relies on mockserver replying faster than start_requests schedules requests or there is something more stable. When I extracted this spider and ran it via scrapy crawlspider on a separately started mockserver it also often (but not always) fails with the same problem. It's possible that the test is just flaky and depends on performance of various components, but I don't remember if it fails on some CI runs.

[codecov]

Codecov Report

Merging #5949 (508b50d) into master (33b418d) will increase coverage by 0.05%.
The diff coverage is 100.00%.

❗ Current head 508b50d differs from pull request most recent head 493ea43. Consider uploading reports for the commit 493ea43 to get more accurate results

@@            Coverage Diff             @@
##           master    #5949      +/-   ##
==========================================
+ Coverage   88.81%   88.86%   +0.05%     
==========================================
  Files         162      162              
  Lines       11184    11199      +15     
  Branches     1819     1821       +2     
==========================================
+ Hits         9933     9952      +19     
+ Misses        965      962       -3     
+ Partials      286      285       -1     

Impacted Files	Coverage Δ
scrapy/exporters.py	100.00% <100.00%> (ø)
scrapy/extensions/feedexport.py	95.80% <100.00%> (+0.16%)	⬆️

... and 2 files with indirect coverage changes

[Gallaecio]

❤️

[wRAR]

Actually test_start_requests_laziness failed in build-time tests too, so whatever it is, it's unrelated to these changes and this mode of running tests, so merging this.