This creates a framework for compressing results as part of the
FileBackend class. This allows for the simple addition of compression
formats (gz is easily possible in python2, xz in python3), but does not
actually implement any compression. zip and bz2 compression are also
possible, although they'll require a little more code.

This patch implements a framework, and tests for that framework.

The goal is that other compression methods can easily be added to piglit
simply by setting a few values in framework/backends/compression, and
then everything will just work.

This will allow junit results that are compressed to be read, but
doesn't add support to junit for compressing results, since junit is
mainly intended for consumption by jenkins, which doesn't understand
compression.

v2: - replace tests decorator set_compression with use of
      utils.set_piglit_conf. This is better because it uses a generic
      tools, and because set_compression is somewhat misleading of a
      name.
v3: - backport changes from my attempt to port this to python3. There
      are significant differences between python2 and python3 compression
      handling because of the bytes/str/unicode differences between the
      two versions. There are also code cleanups from that work here
    - Make a better effort to protect the unit tests from the
      environment around them. The goal is that the tests control the
      compression options 100%, with no chance of piglit.conf or
      environment variables affecting the tests.
    - reduce code duplication in unit tests
    - add additional unit tests as made possible by refactoring changes
v4: - add additional unit tests

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>