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/isNumber.test.js

@@ -0,0 +1,42 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { falsey, args, slice, symbol, realm } from './utils.js';
+import isNumber from '../isNumber.js';
+
+describe('isNumber', function() {
+  it('should return `true` for numbers', function() {
+    assert.strictEqual(isNumber(0), true);
+    assert.strictEqual(isNumber(Object(0)), true);
+    assert.strictEqual(isNumber(NaN), true);
+  });
+
+  it('should return `false` for non-numbers', function() {
+    var expected = lodashStable.map(falsey, function(value) {
+      return typeof value == 'number';
+    });
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      return index ? isNumber(value) : isNumber();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+
+    assert.strictEqual(isNumber(args), false);
+    assert.strictEqual(isNumber([1, 2, 3]), false);
+    assert.strictEqual(isNumber(true), false);
+    assert.strictEqual(isNumber(new Date), false);
+    assert.strictEqual(isNumber(new Error), false);
+    assert.strictEqual(isNumber(_), false);
+    assert.strictEqual(isNumber(slice), false);
+    assert.strictEqual(isNumber({ 'a': 1 }), false);
+    assert.strictEqual(isNumber(/x/), false);
+    assert.strictEqual(isNumber('a'), false);
+    assert.strictEqual(isNumber(symbol), false);
+  });
+
+  it('should work with numbers from another realm', function() {
+    if (realm.number) {
+      assert.strictEqual(isNumber(realm.number), true);
+    }
+  });
+});