channel: Increase grace period for subprocesses to stop

[Rahix]

There were complaints that tbot would raise the "some subprocess(es) did not stop" error too quickly. The previous 1.27s timeout was too short for some programs to exit on their own.

To make tbot more graceful, increase the maximum termination time to 10.2 seconds. However, a warning will be emitted at 1.27s to let the user know why tbot is waiting.

Fixes: #116 Cc: @VictorEmelin

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 22.22222% with 7 lines in your changes missing coverage. Please review.

Project coverage is 62.26%. Comparing base (d2c043f) to head (1e14da5).

Files	Patch %	Lines
tbot/machine/channel/subprocess.py	22.22%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #120 +/- ## ========================================== - Coverage 62.39% 62.26% -0.14% ========================================== Files 53 53 Lines 3755 3747 -8 ========================================== - Hits 2343 2333 -10 - Misses 1412 1414 +2 ```

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