Run Azure integration tests serially under tox

pytest-xdist with -n 5 races against the shared default vnet/subnet:
multiple workers call get_or_create_default cold, all find nothing,
all try to create cloudbridge-net/cloudbridge-subnet, and Azure ARM
rejects all but one with AnotherOperationInProgress or cancels in-
flight create LROs. The losing workers' tests then fail at VM
creation because they have no usable subnet.

Drop the -n 5 default for the azure factor only. Other clouds keep
parallel since their tests don't share singleton default resources
the same way.

tox.ini

@@ -10,7 +10,12 @@ envlist = py3.13-{aws,azure,gcp,openstack,mock},lint
 
 [testenv]
 commands = # see pyproject.toml for coverage options; setup.cfg for flake8
-           coverage run --source=cloudbridge -m pytest -v {posargs:-n 5 tests/}
+           # Azure integration tests race on the shared default vnet/subnet
+           # under xdist (get_or_create_default has no locking, so two workers
+           # creating the same named resource conflict on Azure ARM). Run the
+           # azure factor serially; other providers stay parallel.
+           !azure: coverage run --source=cloudbridge -m pytest -v {posargs:-n 5 tests/}
+           azure: coverage run --source=cloudbridge -m pytest -v {posargs:tests/}
            # Combine parallel-mode data files and emit Cobertura XML for upload
            # by coverallsapp/github-action in CI. Locally this produces
            # coverage.xml in the project root, which IDEs can also consume.