Add the option to tag test cases, and run (or skip) certain tags

[nixpanic]

One of the QA guys reported a bug against Gluster/NFS. The problem was reproducible with quite a time-consuming test:

mount -t nfs storage.example.com:/volume /mnt
cd /mnt
if [ ! -e linux-3.19.tar.gz ]
then
    curl https://cdn.kernel.org/pub/linux/kernel/v3.x/linux-3.19.tar.gz > linux-3.19.tar.gz
fi

# abort on error
set -e

for i in {1..1000}
do
    tar -xvf linux-3.19.tar.gz
    sleep 180
    rm -rf linux-3.19
done

I would like to write a test-case for this, but it should not execute by default. It would be nice to have an option to mark a test-case with "tags: long-running" or something in the doc string so that tests like this can easily be skipped.