Add initial test files from lodash v4. (#4172)

* Install test dependencies.

* Add initial test files.

These files were created using a simplistic AST manipulator using `recast` to
preserve formatting. There's bound to be a huge chunk of errors, but this serves
as a good start. QUnit was replaced with Mocha, with ES2015 imports running via
`esm`.

As far as possible, QUnit-specific syntax has been replaced with Mocha's
`describe` and `it`, while the native Node.js `assert` module is used for
assertions. Files in the `test` directory ending in `.test.js` will be treated
as test files.

* Add initial passing files to test run.

test/toLength.test.js

@@ -0,0 +1,22 @@
+import assert from 'assert';
+import { MAX_INTEGER, MAX_ARRAY_LENGTH } from './utils.js';
+import toLength from '../toLength.js';
+
+describe('toLength', function() {
+  it('should return a valid length', function() {
+    assert.strictEqual(toLength(-1), 0);
+    assert.strictEqual(toLength('1'), 1);
+    assert.strictEqual(toLength(1.1), 1);
+    assert.strictEqual(toLength(MAX_INTEGER), MAX_ARRAY_LENGTH);
+  });
+
+  it('should return `value` if a valid length', function() {
+    assert.strictEqual(toLength(0), 0);
+    assert.strictEqual(toLength(3), 3);
+    assert.strictEqual(toLength(MAX_ARRAY_LENGTH), MAX_ARRAY_LENGTH);
+  });
+
+  it('should convert `-0` to `0`', function() {
+    assert.strictEqual(1 / toLength(-0), Infinity);
+  });
+});