[feat] Asyncio execution policy

[Blanca-Fuentes]

This PR changes the asynchronous execution policy to use asyncio (AsyncioExecutionPolicy).

The main changes introduced are the following:

• KeyBoardInterrupt management: introduction of a new type of exception KeyboardError to raise in case of keyboard interruption during the execution of the code.
• attach_hooks(): if the hook is the run / wait methods of a test then, they need to be treate as asyncio coroutines
• Logging context manager: switching the control within the logging context must be treated carefully. I created a _global_logger and a tasks_logger. The _global_logger is used when the asyncio loop has not been yet opened and the tasks_logger to retrieve the right logging context for each task.
• The stage_dir and the output_dir must be absolute paths for the right context switching with with change_dir_global. The working directory of reframe is now set at the beginning with set_working_dir, it is retrieved at any point in the code with get_working_dir because changing directory with asyncio does not guarantee that we will be in the right directory when doing os.getcwd().
• The test methods compile, run, compile_wait, run_wait are now asynchronous.
• New run_command_asyncio method was added to create an asyncio subprocess, which has the same output as the run_command_async. This method waits (asynchronously) for the command to be executed. The method run_command_async_alone just starts the subprocess but does not wait for completion (this allows the local scheduler to poll for the job completion).
• The termination and killing (_term_all() and _kill_all()) of the processes in the local scheduler is more challenging because the communication of signals between children and parent processes is not handled in the same way. The children of a process are retrieved explicitly and signals are sent to each process individually.
• _run_strict and _run_strict_s are the asyncio and synchronous versions of _run_strict.
• The handling of the different cases (tests) takes place inside execute() not in exit() as in the previous AsynchronousExecutionPolicy.
• Adaptation of the tests for the new asyncio policy.

TODO:

• Extend the implementation to other schedulers (only slurm /local)
• Test performance vs previous AsynchornousExecutionPolicy
• Create more tests for the unittests

[pep8speaks]

Hello @Blanca-Fuentes, Thank you for submitting the Pull Request!

• In the file reframe/core/exceptions.py:

Line 113:1: E302 expected 2 blank lines, found 1

• In the file reframe/frontend/executors/policies.py:

Line 648:80: E501 line too long (80 > 79 characters)
Line 665:80: E501 line too long (82 > 79 characters)

• In the file reframe/utility/osext.py:

Line 374:1: E302 expected 2 blank lines, found 1

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