tox.ini:
 Generate a Junit xml report to `test-reports/junit.xml`.  Generate
 coverage reports test-reports/coverage.xml, test-reports/htmlcov,
 and the "TOTAL" line of the report to stdout (for consumption
 by gitlab CI).

.pipeline/blubber.yaml:
 Changed the 'test' variant so that it runs tox at image build time,
 so if the image build completes successfully, tests have passed.
 Added test-<distro>-report variants which copy only the test-reports
 directory.

.gitlab-ci.yml:

 Changed the `test` job to use build-image with the
 test-<distro>-report variants, exporting the image to a local
 directory when the image completes.  The test results (junit.xml) and
 coverage report is exported as a GitLab CI artifact for integrated
 viewing in MRs.

Makefile:
 Remove --quiet flag from the `docker build` of a test image,
 so that the test output can be seen.

Change-Id: Ic63b016d5c29eebb393e5cd3cc63b7626ea197b5