.gitlab-ci.yml: a demo pipeline with allow_failure

see #27
4 jobs for demo/fail-a in 23 seconds (queued for 6 minutes and 25 seconds)
latest
Status Job ID Name Coverage
  Unit
passed #10261
unit

00:00:02

 
  Demo
passed #10262
demo

00:00:06

 
  Demo A
failed #10263
allowed to fail
demo_a

00:00:06

 
  Coverage
passed #10264
coverage

00:00:07

86.0%
 
Name Stage Failure
failed
demo_a Demo A
    x.append(test.xml())
File "src/should.py", line 564, in xml
x = ET.Element('testcase', {'name': self.name, 'status': self.status.xml()})
AttributeError: 'str' object has no attribute 'xml'
Uploading artifacts...
Runtime platform arch=386 os=linux pid=15756 revision=58272c27 version=12.7.0
should.xml: found 1 matching files

Uploading artifacts to coordinator... ok
id=10263 responseStatus=201 Created token=eBJCfaco
ERROR: Job failed: exit status 1