Support for pdb on process spawner

[richtja]

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Right now, debugging of avocado python based tests with pdb is not possible because of spawner architecture. If you run avocado tests with pdb the only part which will be tracked is avocado core. This is not convenient from user's perspective when you want to track the test itself. This is caused by spawner architecture where the actual tests are spawned in different environment where the pdb is not enabled. Let's add native support for pdb in avocado process spawner.

Describe the solution you'd like
A clear and concise description of what you want to happen.
Imagine avocado instrumented test with breakpoints. Let's add ability to run this test through pdb:

$ python3 -m pdb avocado run test.py

Describe alternatives you've considered
Another possible solutionis not that intuitive, but it will be much easier to make. For users, generate a command for running failed test with runner and such test should be able to run with pdb:

$ python3 -m pdb avocado-runner-avocado-instrumented task-run -i=1 -u=test.py:TimeoutTest.test