devguide/testing: much refactoring and editing

- improve wording
- add links
- new BDD and web UI pages
- remove duplicate information

and more ...
dev: tidy up handling of test environment prerequisites

- Move all handling of test environment dependencies into a single function
- Rename that function to test_prereqs_installed()
- Fix DRY violations by removing gem dependencies from ./dev and
  dev-setup/Gemfile and always using bundler to install them via the
  existing Gemfile.  This reduces the chance of us missing a gem during
  testing due to the minimal Gemfile previously used being too minimal.
- Remove pointless 'f' temporary variable
devguide: fix test environment crowbar_framework path

96b62ac094 broke the docs :-/
pkill is a more robust way of killing processes

The previous code caused a warning when no matching PID was found.
auto-reloada environment for BDD tests if necessary
devguide: suggest running BDD tests via ./dev