feat: add /bugbot test to probot client

[ckerr]

Some of the groundwork has already been done:

• feat: flexible commands in issue comments #100 added prerequisite code for parsing /bugbot test commands. Currently doesn't do anything with them. (merged)
• feat: add code to pick Electron versions to test #96 adds prerequisite code to figure out what Electron versions to use in the matrix. (ready for review)
• Probot Client should be able to update its own comment #97 will add prerequisite code for overwriting an issue comment so we can update the matrix when new information comes in.

Now we need code to tie all these pieces together.

• GithubClient needs to kick of N test jobs, maintain state for >1 job per issue, keep polling until done, and periodically update the matrix with the latest data.