[lldb-dap] Refactoring DebugCommunication to improve test consistency. (#143818)

In DebugCommunication, we currently are using 2 thread to drive
lldb-dap. At the moment, they make an attempt at only synchronizing the
`recv_packets` between the reader thread and the main test thread. Other
stateful properties of the debug session are not guarded by a
locks/mutex.

To mitigate this, I am moving any state updates to the main thread
inside the `_recv_packet` method to ensure that between calls to
`_recv_packet` the state does not change out from under us in a test.

This does mean the precise timing of events has changed slightly as a
result and I've updated the existing tests that fail for me locally with
this new behavior.

I think this should result in overall more predictable behavior, even if
the test is slow due to the host workload or architecture differences.

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Co-authored-by: Ebuka Ezike <[email protected]>

Author: ashgti