[Bug] Using the shard and coverage cli options together shouldn't output instanbul report

[bryanjtc]

Describe the bug

By default the coverage cli option outputs a instanbul text report each time the test-runner is ran.
When using the shard cli option, a custom test sequencer is needed. This means that the test-runner is running in multiple ci jobs, each job with a different set of tests. After the tests have executed, the coverage results should be merge and then the nyc command should run to output the complete instanbul report.

Steps to reproduce the behavior

1. Add a custom test sequencer
2. Use the coverage ans shard cli options together.