Merge branch 'selftests-tc-testing-parallel-tdc'

Pedro Tammela says:

====================
selftests/tc-testing: parallel tdc

As the number of tdc tests is growing, so is our completion wall time.
One of the ideas to improve this is to run tests in parallel, as they
are self contained.

This series allows for tests to run in parallel, in batches of 32 tests.
Not all tests can run in parallel as they might conflict with each other.
The code will still honor this requirement even when trying to run the
tests over the worker pool.

In order to make this happen we had to localize the test resources
(patches 1 and 2), where instead of having all tests sharing one single
namespace and veths devices each test now gets it's own local namespace and devices.

Even though the tests serialize over rtnl_lock in the kernel, we
measured a speedup of about 3x in a test VM.
====================

Link: https://lore.kernel.org/r/20230919135404.1778595-1-pctammela@mojatatu.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>