Improve test suite: increase coverage and reduce total execution time

[vdusek]

Summary

Umbrella issue tracking improvements to the test suite — adding new tests, restructuring existing ones, increasing code coverage, and reducing execution time.

Related PRs

• test: split integration tests into integration and e2e suites #781

• test: Increase total test coverage and reduce E2E tests #783

• test: add e2e tests for Crawlee crawlers as Apify Actors #784

• test: add e2e tests for Scrapy spiders running as Apify Actors #788

• https://github.com/apify/apify-core/pull/25840

• ci: Remove E2E tests concurrency group to allow parallel matrix jobs #811

[vdusek]

Addressed in #789 — adds a global concurrency group to ensure only one E2E test job runs at a time across all workflow runs.