tests: avoid leaks into host system checking of ovs-vsctl cmd

[blackboxsw]

Proposed Commit Message

tests: avoid leaks into host system checking of ovs-vsctl cmd

Mock out the `which` checks for presence of ovs-vsctl cmd on system. This prevents test leaks which eventually call subp ovs-vsctl commands.

Additional Context

Test Steps

# install openvswitch-switch
sudo apt install openvswitch-switch
# Run failing unit tests:
tox -e py3 tests/unittests/test_net.py



FAILED tests/unittests/test_net.py::TestGenerateFallbackConfig::test_device_driver - RuntimeError: called subp. set self.allowed_subp=True to allow
FAILED tests/unittests/test_net.py::TestGenerateFallbackConfig::test_hv_netvsc_vf_filter - RuntimeError: called subp. set self.allowed_subp=True to allow
FAILED tests/unittests/test_net.py::TestNetplanNetRendering::test_render[default_generation] - AssertionError: Unexpectedly used subp.subp
============================================= 3 failed, 4777 passed, 4 skipped, 1 deselected, 1 xfailed, 271 warnings in 68.30s (0:01:08) ====

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]

LGTM!