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/mergeWith.js

@@ -0,0 +1,64 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { noop, identity, isNpm, mapCaches } from './utils.js';
+import mergeWith from '../mergeWith.js';
+import last from '../last.js';
+
+describe('mergeWith', function() {
+  it('should handle merging when `customizer` returns `undefined`', function() {
+    var actual = mergeWith({ 'a': { 'b': [1, 1] } }, { 'a': { 'b': [0] } }, noop);
+    assert.deepStrictEqual(actual, { 'a': { 'b': [0, 1] } });
+
+    actual = mergeWith([], [undefined], identity);
+    assert.deepStrictEqual(actual, [undefined]);
+  });
+
+  it('should clone sources when `customizer` returns `undefined`', function() {
+    var source1 = { 'a': { 'b': { 'c': 1 } } },
+        source2 = { 'a': { 'b': { 'd': 2 } } };
+
+    mergeWith({}, source1, source2, noop);
+    assert.deepStrictEqual(source1.a.b, { 'c': 1 });
+  });
+
+  it('should defer to `customizer` for non `undefined` results', function() {
+    var actual = mergeWith({ 'a': { 'b': [0, 1] } }, { 'a': { 'b': [2] } }, function(a, b) {
+      return lodashStable.isArray(a) ? a.concat(b) : undefined;
+    });
+
+    assert.deepStrictEqual(actual, { 'a': { 'b': [0, 1, 2] } });
+  });
+
+  it('should provide `stack` to `customizer`', function() {
+    var actual;
+
+    mergeWith({}, { 'a': { 'b': 2 } }, function() {
+      actual = last(arguments);
+    });
+
+    assert.ok(isNpm
+      ? actual.constructor.name == 'Stack'
+      : actual instanceof mapCaches.Stack
+    );
+  });
+
+  it('should overwrite primitives with source object clones', function() {
+    var actual = mergeWith({ 'a': 0 }, { 'a': { 'b': ['c'] } }, function(a, b) {
+      return lodashStable.isArray(a) ? a.concat(b) : undefined;
+    });
+
+    assert.deepStrictEqual(actual, { 'a': { 'b': ['c'] } });
+  });
+
+  it('should pop the stack of sources for each sibling property', function() {
+    var array = ['b', 'c'],
+        object = { 'a': ['a'] },
+        source = { 'a': array, 'b': array };
+
+    var actual = mergeWith(object, source, function(a, b) {
+      return lodashStable.isArray(a) ? a.concat(b) : undefined;
+    });
+
+    assert.deepStrictEqual(actual, { 'a': ['a', 'b', 'c'], 'b': ['b', 'c'] });
+  });
+});