[docs] document how to run osx tests

using virtualbox executor for gitlab-runner
12 jobs from master in 102 minutes 52 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #29095
docker-in-docker
build_docker_image

00:39

passed #29093
linux
linux_test

12:37

passed #29094
osx yosemite allowed to fail
osx_test

06:45

 
  E2e
passed #29096
linux allowed to fail
e2e_tests

06:49

 
  Functional
passed #29097
linux
functional_tests

08:06

 
  Bundle
passed #29098
linux
bitmask_latest_bundle

08:03

 
  Build Ui
passed #29099
linux
build_ui

01:36

 
  Package
passed #29104
package:amd64_artful

32:33

failed #29101
package:amd64_buster

01:00:04

failed #29102
package:amd64_sid

01:00:04

passed #29100
package:amd64_stretch

54:31

passed #29103
package:amd64_zesty

59:46

 
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