Run all found test frameworks, rather than just one

[Gedochao]

Fixes #3620

What this does:

• on the JVM, all test frameworks should be detected and ran automatically
• on Scala Native & Scala.js platforms, all recognised test frameworks should be detected and ran automatically
  • unknown (non-recognised) and custom test frameworks are limited to 1 at a time, and not together with others; this is due to deep-searches for unknown test frameworks being very time-consuming;
  • this is still a straight-up upgrade over the past behaviour, when a single test framework would be run at all times.
• the --test-framework CLI option and //> using testFramework using directive work as before, where only the specified test framework is to be run
  • note that this gives the best performance, particularly for Scala Native & Scala.js tests, as this effectively skips test framework detection and runs with what is passed

Potential future follow-ups:

• handle multiple test frameworks on SBT/Mill export: Handle multiple test frameworks when exporting to SBT/Mill #3634
• expand the --test-framework CLI option and //> using testFramework directive to accept multiple values, allowing to preconfigure multiple frameworks and skipping detection: Allow to preconfigure multiple test frameworks, rather than just one. #3632
• add a deep search flag, which would allow someone to actually deep search for test frameworks on Scala Native & Scala.js, if they really care to and don't mind the long loading times (not sure how many use cases that'd have, but it doesn't hurt to add a flag)
• add a --print-test-frameworks flag, similar to --print-classpath, to make it easier to find what test frameworks are available in the project: Add a flag for printing all available test frameworks in the project #3633
• add a global config for preconfiguring more recognised test frameworks

[tgodzik]

Looks good! I just added some comments, but feel free to ignore me.