Allow testing without downloading resources - for Debian building about imageio

[almarklein]

cc @ghisvail

[almarklein]

Copy from mailing list, for easy reference when I am going to work on this:

I'll walk you through the package build process.

It takes an upstream tarball, like the v1.0 available on PyPi, and add a debian/
folder to it which contains all the necessary meta-data and rules to build the pkg.

For Python pkg, we have a helper which automatically cleans, configures, builds,
install and tests the pkg for all supported Python versions (now 2.7 and 3.4, but
later 3.4+). For the test suite, the build system autodetects how the test suite is
implemented, for instance with a separate test/ or tests/ folder, and runs a
discover to it. In your case, there is a testing.py in the source tree, which the
build system seems to be trying to run.

You are right though, if you do not ship the test suite with the source release, then
there is no reason to run it. However, Debian is encouraging us, pkg maintainers to
convince upstream devs, like yourself, to ship the test suite as well, so the latter
can be used for continuous integration purposes at the distribution level.

Again, this is entirely up to you and is not a pressing issue. It is only a
recommendation for now, though I foresee it to become more and more "encouraged",
to the point where a working test suite will be a requirement for any package in some
future.

Please let me know whether this was detailed enough.