Add .gitlab-ci.yml: tests and style checking

Signed-off-by: Jerome Flesch <jflesch@openpaper.work>
2 jobs for <span class="ref-name">wip-ci</span> in 7 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #297262
volatile linux
check

00:05:58

passed #297263
volatile linux
test

00:07:33