Get core and wagon tests to work reliably, various improvements

[severinraez]

See also https://github.com/hitobito/hitobito/pull/1468

• Use correct ruby version for tests
• Add setup for core tests, add scripts bin/test_*, update README
• Fix wagon tests
• Move wagonfile template to hitobito so local developers use the same setup
• Add bin/bundle_show, bin/dump_database, bin/load_database, bin/dbconsole
• Speed up entrypoints
• Add 'less' to image
• Update bash prompt when changing branch
• Simplify debugging
  • Allow skipping entrypoint
  • Do not wrap command in bundle exec so docker-compose exec behaves like docker-compose run

Refs https://github.com/hitobito/hitobito/issues/1463

Progress