Run TF and DL tests without restarting process

[ry]

Currently to switch backends, we need to restart Node with different environmental variables:

propel/tools/test.js

Lines 9 to 11 in 8b570f0

	// Node.js tests
	run.tsnode("tools/test_node.ts", {"PROPEL": "dl"}); // DL backend
	run.tsnode("tools/test_node.ts", {"PROPEL": "tf"}); // TF backend

This could be done during runtime by improving this bit of code

propel/src/backend.ts

Lines 27 to 48 in 8b570f0

	// These globals will be set by onLoad
	export let backend: string;
	export let bo: types.BackendOps;
	let onLoadCalled = false;
	(function onLoad() {
	if (onLoadCalled) {
	console.warn("Warning: backend.onLoad called more than once.");
	return;
	}
	onLoadCalled = true;
	if (preferTF() && tf.loadBinding()) {
	console.log("Using TF backend.");
	bo = new tf.OpsTF();
	backend = "tf";
	} else {
	console.log("Using DL backend.");
	bo = new dl.OpsDL();
	backend = "dl";
	}
	})();

Once we can run both backends without restarting the process, it will be possible to augment the tester to run tests for both. That would be done here:

propel/tools/tester.ts

Lines 49 to 59 in 8b570f0

	export function test(t: TestDefinition | TestFunction): void {
	const fn: TestFunction = typeof t === "function" ? t : t.fn;
	const name: string = t.name;
	if (!name) {
	throw new Error("Test function may not be anonymous");
	}
	if (filter(name)) {
	tests.push({ fn, name });
	}
	}