machine: linux: Add sanity check for shell interaction

[Rahix]

Add a sanity check that ensures the shell on the remote end is really in the state we expect it to be in before we start interacting with it.

This check should uncover the following kinds of errors early:

• Heavy amounts of output clobbering from some source
• tbot tracking the wrong prompt (offset error)
• fancy shell configuration that tbot wasn't able to disable

Previously, such errors would only surface later, when trying to run commands. The error messages were quite confusing and didn't immediately point to the root cause. This sanity check should hopefully make debugging these issues much easier.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 62.46%. Comparing base (0745f2c) to head (36fb7a2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #115 +/- ## ========================================== + Coverage 62.35% 62.46% +0.11% ========================================== Files 53 53 Lines 3735 3746 +11 ========================================== + Hits 2329 2340 +11 Misses 1406 1406 ```

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