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/flatMapDepth.js

@@ -0,0 +1,33 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { identity } from './utils.js';
+import flatMapDepth from '../flatMapDepth.js';
+
+describe('flatMapDepth', function() {
+  var array = [1, [2, [3, [4]], 5]];
+
+  it('should use a default `depth` of `1`', function() {
+    assert.deepStrictEqual(flatMapDepth(array, identity), [1, 2, [3, [4]], 5]);
+  });
+
+  it('should use `_.identity` when `iteratee` is nullish', function() {
+    var values = [, null, undefined],
+        expected = lodashStable.map(values, lodashStable.constant([1, 2, [3, [4]], 5]));
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? flatMapDepth(array, value) : flatMapDepth(array);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should treat a `depth` of < `1` as a shallow clone', function() {
+    lodashStable.each([-1, 0], function(depth) {
+      assert.deepStrictEqual(flatMapDepth(array, identity, depth), [1, [2, [3, [4]], 5]]);
+    });
+  });
+
+  it('should coerce `depth` to an integer', function() {
+    assert.deepStrictEqual(flatMapDepth(array, identity, 2.2), [1, 2, 3, [4], 5]);
+  });
+});