remove vcr tests because hooking into webmock breaks multithreaded tests

[grosser]

#494 breaks when VCR is enabled .. somehow it ends up executing the stubs on all threads

can reproduce by adding to test/test_helper.rb

require 'vcr'
VCR.configure do |c|
  c.cassette_library_dir = 'test/cassettes'
  c.hook_into(:webmock)
end

and running ruby test/test_informer.rb

the guestbook test itself seems rather brittle / not reproducible, so I'm thinking best get rid of it instead of pouring more work into it

[cben]

I guess you're right.

I was recently thinking to automate some version of k8s that's easy to start, e.g. k3s, to be able to reproducibly re-record it the VCR. But if we make something like that, we don't need VCR, we could get real end-to-end testing. As it is, the VCR test is not worth much.

[cben]

Don't know yet what's going on with truffleruby. Added it to #435 checklist so we don't forget. This PR is obviously safe, merging.