Co-authored-by: Michael Moravec <me@majkl.me>
Co-authored-by: Benjamin Morel <benjamin.morel@gmail.com>

Currently, builds like https://travis-ci.org/doctrine/dbal/jobs/440354436 are failing do to an APT issue. Trying to update APT packages as per support recommendation.

1. Use the actual dsdriver distribution
2. Introduced `AbstractPlatform::getCharMaxLength()` since `CHAR` and `VARCHAR` limitations are different on DB2.

1. XSD location pointing to the vendored file.
2. Removed `backupGlobals="false"`.
3. Added `failOnRisky="true" failOnWarning="true"`.
4. Added `beStrictAboutOutputDuringTests="true" beStrictAboutTodoAnnotatedTests="true"`.
5. Added code coverage whitelist.

I am not sure if there is a rationale behind ignoring extra keys like
this. I was under the impression that something might copy options to
platform specific options, thus leaving options in the original array,
knowing they would be ignored anyway. If it is the case, it would be
best to just move them, otherwise, this commit might help troubleshoot
some mapping issues.
I had to fix some tests that wrongly used that options array (maybe
these options were valid at some point?).

This is now mandatory on the new Travis-CI containers.