test: Convert some tests to Typescript

[gonfunko]

The basics

• I validated my changes

The details

Proposed Changes

This PR converts several core test files and helpers to Typescript. Generally, this entails:

• Renaming the files
• Declaring locally scoped variables for workspaces or other objects created in setup() and updating tests to reference those rather than accessing them on this
• Explicitly passing workspaces created in setup() to the teardown methods
• Adding types to helper functions

A few tests were modified or dropped:

• The "check character -> constant" test from field_checkbox_test was removed because it was mutating a readonly property, which has been unsupported for some time
• The serializeKeyEvent suite from shortcut_registry_test was dropped, because that method is private
• The oldxml test from trashcan_test was removed, because the oldXml field was removed from events in v13
• Two tests that exercised passing null for an argument that is typed as undefined were removed from variable_model_test and variable_map_test; there were already tests for the undefined case, which were left in place.
• The domToWorkspace suite in xml_test was largely passing by accident; all the assert.throws() were throwing (and thus passing) because this.workspace was undefined, as the function passed to the assert was not an arrow function. Switching to locally-scoped variables fixed that, but made the "Variables at top missing type" test start failing, because the exception it was expecting had been removed from core long ago; I updated that test to assert that it does not throw.

There are also two small core changes:

• A TEST_ONLY export was removed from toolbox.ts because the sole use of it in a test was rewritten to use supported public APIs
• The argument type for initializeGlobalAriaLiveRegion() was broaded from HTMLDivElement to HTMLElement; nothing about the implementation actually cares that the element is a <div> specifically.

Test Coverage

All tests pass, now with fewer bugs.