tests: mock LXD datasource detection in ds-identify on LXD containers

[blackboxsw]

Proposed Commit Message

On LXD containers /dev/lxd/sock will always exist.

Mock dscheck_LXD to return 1 (NOT_FOUND) to avoid leaking into
test environment and returning LXD as detected.

We have integration tests covering proper LXD datasource detection
so we don't need a Unit test validating the [ -S /dev/lxd/sock] that
is in ds-identify.

Additional Context

Test Steps

lxc launch ubuntu-daily:bionic dev-b
lxc config device add dev-b rohome disk source="/home/csmith/src/cloud-init" path="/root/cloud-init/"
lxc exec dev-b bash
tox -e py3 tests/unittests/test_ds_identify.py

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[TheRealFalcon]

Thanks!