test(testrun): cover the sidecar shutdown path after an early exit

internal/testrun/driver.go

@@ -166,13 +166,7 @@ func buildDriver(ctx context.Context, options Options, stdout io.Writer) (driver
 	if err := sidecarCommand.Start(); err != nil {
 		return nil, nil, fmt.Errorf("spawn sidecar: %w", err)
 	}
-	// The close after the send lets the cleanup path receive again once the
-	// startup path has already taken the exit status.
-	sidecarExited := make(chan error, 1)
-	go func() {
-		sidecarExited <- sidecarCommand.Wait()
-		close(sidecarExited)
-	}()
+	sidecarExited := watchSidecar(sidecarCommand)
 	address := fmt.Sprintf("127.0.0.1:%d", sidecarPort)
 	fmt.Fprintf(stdout, "sidecar pid=%d listening on %s (adb: %s)\n", sidecarCommand.Process.Pid, address, adbPath)
 
@@ -205,6 +199,18 @@ func buildDriver(ctx context.Context, options Options, stdout io.Writer) (driver
 	return driverClient, cleanup, nil
 }
 
+// watchSidecar reaps the sidecar and publishes its exit status. The channel is
+// closed after the send so the shutdown path can still receive once the startup
+// path has taken the status.
+func watchSidecar(sidecarCommand *exec.Cmd) <-chan error {
+	exited := make(chan error, 1)
+	go func() {
+		exited <- sidecarCommand.Wait()
+		close(exited)
+	}()
+	return exited
+}
+
 // awaitSidecar waits for the sidecar to answer a health check, racing that
 // against the process exiting so a sidecar that dies during startup is reported
 // as the exit it was rather than as a deadline half a minute later. Neither