Ignore pylint 1.x errors related to GObject Introspection

pylint 1.1.0 raises all sorts of E1101 (no-member) errors around classes
from gi (GObject Introspection). This is in spite of pylint-brain code
that specifically attempts to understand GObject Introspection.[1]
In theory the pylint-brain code should instead read the introspection
data from the GI registry and provide dummy classes with all the right
members.

This commit just turns off pylint for any GObject or GStreamer classes.
It isn't ideal, but at least `make check-pylint` is *somewhat* useful
again.

Furthermore python's inspect.py raised a bunch of GObject-related
warnings, presumably to do with the modules imported by pylint-brain.
The warnings obscured the output of `make check` so this commit hides
those warnings.

[1] https://bitbucket.org/logilab/pylint-brain/src/6785d1aa/brain/py2gi.py
run-tests.sh: Ensure that environment variables are set when sourcing tests

I ran into an issue where I had something like:

    some_file="$testdir/filename"

in my test script.  This was being evaluated at test discovery time so the
filename was not what I was expecting.  This commit fixes this issue by
exporting those constant test variables before doing test discovery.

This restores the behaviour from before 4843875.