We currently have two sites file containing redundant information.

- tests/testutils/site.py: Remove IS_{LINUX,WSL,WINDOWS}
- rest: Change imports to import those from
buildstream/tests/_utils/site.py

We have two different 'site' files that are redundant and both
define some variables in BuildStream environment.

Moving the MACHINE_ARCH to a single place.

We have two different 'site' files that are redundant and both define
some variables in BuildStream environment.

Moving all the bubblewrap related ones in a single place.

- Rename plugintestutils to testing.

- Don't run the tests from bst-plugins-template. This imports
  buildstream.plugintestutils so will have to be disabled to get
  through CI. This can be re nabled once bst-plugins-template has been
  patched.

Replace some popular copypasta.

This important-looking invocation:

    os.path.join(datafiles.dirname, datafiles.basename)

is equivalent to this shorter invocation:

    project = str(datafiles)

It seems like it's very popular copypasta, replace it with the shorter
one thus:

    # Use 'gsed' or 'sed' etc. as appropriate for your system.
    git config --global alias.sub '!f() { git grep --name-only --null "$1" | gxargs --null gsed --in-place --expression "s/$1/$2/g" ; }; f'
    git sub 'os.path.join(datafiles.dirname, datafiles.basename)' 'str(datafiles)'

Pylint can't know that pytest's fixtures are used in a file and
therefore reports false positives. Silencing all those errors

Pylint doesn't play well with pytest fixtures, we therefore need to
silence this error.