tests: Improve unit tests runtime performance

This patch set does 2 things:

1) Improves unit test runtime peformance via pytest-xdist [0]
2) Reduces finnicky nature of `is_connected` helpers which
   sometimes skip even when there is access to the internet;
   logic has been added to make these checks more accurate
   to avoid skipping tests

Note that while there are newer alternatives to pytest-xdist they
are only compatible with much newer versions of Python.

[0] https://pypi.org/project/pytest-xdist/

Change-Id: Ib04b48ebabca0551058e5e1065056f4e559fbfe6

tools/gate/run-unit-tests.sh

@@ -2,9 +2,11 @@
 
 set -e
 posargs=$@
+# cross-platform way to derive the number of logical cores
+readonly num_cores=$(python -c 'import multiprocessing as mp; print(mp.cpu_count())')
 if [ ${#posargs} -ge 1 ]; then
-    pytest -k  ${posargs}
+    pytest -k ${posargs} -n $num_cores
 else
-    pytest
+    pytest -n $num_cores
 fi
 set +e