Migrate existing comm tests to protocol-level tests

[lionel-]

In recent PRs we've started using the DummyArkFrontend infrastructure for protocol-level tests of comms:

• Synchronise Positron's Variables pane with debug environment #1052 (variables)
• Notify Variables pane when frame is selected #1060 (data explorer and variables)
• Add plot file/origin attribution #1067 (plots)

These tests are much nicer and effective than the old comm-based legacy tests because:

• They exercise the whole kernel. This means that there is no need to simulate environment changed events after modifying the global env, which requires knowing a deeper knowledge of how internals work to produce a meaningful test (see

  ark/crates/ark/tests/variables.rs

  Line 116 in 239ee7e

  EVENTS.environment_changed.emit(());

  ). We can let the normal operation of the kernel generate this sort of events.

• It allows testing comms in conjunction with DAP and LSP operations and state. For instance the DAP now triggers comm-visible events, and it's nice to exercise these paths.

• Inputs to the tests can be modelled after user requests, such as executing R code in the console. Compare to how we simulate global environment changes for Variables pane tests:

  ark/crates/ark/tests/variables.rs

  Lines 109 to 113 in 239ee7e

  	r_task(|| unsafe {
  	let test_env = test_env.get().clone();
  	let sym = r_symbol!("everything");
  	Rf_defineVar(sym, Rf_ScalarInteger(42), *test_env);
  	});

Before doing that though, we should solve #689. This will allow comms to behave deterministically and tested with sequential assertions that are much stronger than fuzzy matching of events.