Clean up Pilot acquisition.

Clarify some naming.
Consolidate logic from conftest and elsewhere for creation and maintenance of the scalems.radical.runtime.RuntimeSession.
Use a creation function to provide RuntimeSession with an asyncio event loop.
Update the testing framework to use the new encapsulated RuntimeSession lifetime management.

Note: We need to interact with the RuntimeSession in the main thread whenever possible. Let the RuntimeSession dispatch slow rp UI calls to other threads as needed and appropriate.

TODO(#334):

Avoid using an extra thread. Use a Pilot callback and asyncio logic to get Pilot runtime resources dict.
When we use to_thread for rp UI, we MUST schedule watcher that makes sure the thread is freed a reasonable amount of time after canceling the asyncio Future wrapper.

TODO(#335):

Break up the scalems.radical.runtime module for the major classes and their supporting functions and module variables.

Codecov Report

Patch coverage: 66.06% and project coverage change: -1.23 :warning:

Comparison is base (41b965a) 81.04% compared to head (c59f41c) 79.81%.

:exclamation: Current head c59f41c differs from pull request most recent head 04bbe3c. Consider uploading reports for the commit 04bbe3c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #357 +/- ## ========================================== - Coverage 81.04% 79.81% -1.23% ========================================== Files 30 30 Lines 3361 3429 +68 Branches 513 523 +10 ========================================== + Hits 2724 2737 +13 - Misses 465 512 +47 - Partials 172 180 +8 ``` | [Impacted Files](https://app.codecov.io/gh/SCALE-MS/scale-ms/pull/357?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SCALE-MS) | Coverage Δ | | |---|---|---| | [src/scalems/radical/runtime.py](https://app.codecov.io/gh/SCALE-MS/scale-ms/pull/357?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SCALE-MS#diff-c3JjL3NjYWxlbXMvcmFkaWNhbC9ydW50aW1lLnB5) | `74.11% <65.43%> (-5.40%)` | :arrow_down: | | [src/scalems/execution.py](https://app.codecov.io/gh/SCALE-MS/scale-ms/pull/357?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SCALE-MS#diff-c3JjL3NjYWxlbXMvZXhlY3V0aW9uLnB5) | `76.89% <100.00%> (ø)` | |

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SCALE-MS / scale-ms

Clean up Pilot acquisition. #357

Codecov Report