Emit testCase messages before test run starts

[davidjgoss]

Currently cucumber-js emits testCase messages right before each test case will be run. The CCK defines that they should be emitted together at the start of the test run.

(We're temporarily reordering the envelopes in our CCK test instrumentation at the moment.)

[davidjgoss]

Consensus from wider group on this was that the ordering around these messages is not important - other implementatons also reorder their messaged to match CCK

[davidjgoss]

Formatters that want to understand the progress of the test run are best served by emitting testCase messages all upfront, and with pickleAccepted and pickleRejected gone we also need it to solve #1579 nicely.