Support initialization of test context

[jstokes]

Tests always start with an empty context, occasionally you want some information available in the context from the start of a test. It's possible to create a dummy step to fill in these context values as:

#::step{:name 'filling-context
        :title "Filling context values"
        :inputs {}
        :test (constantly "my-output-value")
        :output :foo/bar}

Which creates a context of {:foo/bar "my-output-value"} available to all subsequent steps.

[jstokes]

(deftest with-initial-context
  {::test/description "Test with initial context"
   ::test/context {:a 42}})