Organize Bamboo's TestAdapter test file (#601)

What changed?
============

I wanted to get a better understanding of Bamboo's test helpers to
improve them, but it was difficult to find out how we were testing them
in the `test_adapter_test.exs` .

They all seemed interspersed without any structure. So this commit
organizes them by function they are testing. We also try to use
`assert_raise` were possible to simplify the assertions, while
continuing to match the error and the error message.