The Travis tests don't work due to a Travis networking issue reaching
some external hosts.
Until it is fixed, remove references from the documentation, since its
output is misleading.
We also allow it to fail, which makes the GitHub UI not show commits as
having failed tests because of Travis.
Since the previous commit, GitLab CI does almost all the same tests, so
there should be no significant loss of coverage anyway.
This patch replaces test/README, which was becoming a bit outdated, with
a more general description of the different tests at a high level, and
includes it in the documentation index.
