test: Check if provisionerd is closed before setting run chan (#1145)

This race can be seen here:
https://github.com/coder/coder/runs/6159662393?check_suite_focus=true

provisionerd/provisionerd.go

@@ -230,6 +230,9 @@ func (p *Server) acquireJob(ctx context.Context) {
 	if job.JobId == "" {
 		return
 	}
+	if p.isClosed() {
+		return
+	}
 	ctx, p.jobCancel = context.WithCancel(ctx)
 	p.jobRunning = make(chan struct{})
 	p.jobFailed.Store(false)