test: Refactor mocha test infrastructure

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• Fixes Look into using imports for mocha tests #5468
• Fixes Stop importing node modules directly in Mocha webdriver tests #9735

Proposed Changes

This PR refactors the Mocha test infrastructure. This change was LLM assisted with extensive manual review and edits. Reviewing commit by commit is recommended.

At a high level, this PR makes CI run the Mocha tests under node, and makes npm run test:mocha:interactive more convenient for day to day use by devs. More specifically:

• A bunch of tooling that served to communicate tests results back from the browser to the CLI has been removed; when tests are run under Node in CI, that all just gets outputted directly, and now that the interactive in-browser version is meant for humans, and doesn't need to copy the test results from the browser to the shell.
• Removing that infrastructure removes the need for Webdriver's involvement in the in-browser tests.
• When running the Mocha tests under Node, passing test output is suppressed and only failures are reported.
• The in-browser test page now opens in your default browser, and the full test suite passes under Chrome, Firefox and Safari.
• Edits to tests and Blockly's source both hot-reload the in-browser test suite.
• Tests are found dynamically and bundled with sinon, chai, and locally-built Blockly, so dependencies are resolved via node rather than hardcoded paths to node_modules. This bundling/compilation step also lays the groundwork for writing core tests in Typescript. This did require a new dev dependency on esbuild, but (a) that has no transitive dependencies itself and (b) is very widely used, so I think it's reasonable.
• Various dead code and a gulp-related NPM package have been removed, as they existed in test_tasks.js only to support an old test that measured Blockly's build size and was removed some time ago.

Mocha is copied from its resolved path in node_modules into the build/test directory and loaded from there. Mocha distributes pre-built browser versions, and their own guidance indicates that these should be used. Because the path is resolved, we don't hardcode node_modules anywhere.

Reason for Changes

This improves the speed and reliability of CI, since it no longer runs the test in a browser, improves developer experience when running tests with hot reload, improves reliability of tests (once we start moving them to Typescript), and removes a lot of complex code and hacks (importing modules by path).

Test Coverage

Tests continue to pass, both in the browser and in Node.

[maribethb]

I have some slight hesitations about this but the end result is probably worth it and overall a net simplification.