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/compact.js

@@ -0,0 +1,42 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { LARGE_ARRAY_SIZE, _, falsey } from './utils.js';
+import compact from '../compact.js';
+import slice from '../slice.js';
+
+describe('compact', function() {
+  var largeArray = lodashStable.range(LARGE_ARRAY_SIZE).concat(null);
+
+  it('should filter falsey values', function() {
+    var array = ['0', '1', '2'];
+    assert.deepStrictEqual(compact(falsey.concat(array)), array);
+  });
+
+  it('should work when in-between lazy operators', function() {
+    var actual = _(falsey).thru(slice).compact().thru(slice).value();
+    assert.deepEqual(actual, []);
+
+    actual = _(falsey).thru(slice).push(true, 1).compact().push('a').value();
+    assert.deepEqual(actual, [true, 1, 'a']);
+  });
+
+  it('should work in a lazy sequence', function() {
+    var actual = _(largeArray).slice(1).compact().reverse().take().value();
+    assert.deepEqual(actual, _.take(compact(slice(largeArray, 1)).reverse()));
+  });
+
+  it('should work in a lazy sequence with a custom `_.iteratee`', function() {
+    var iteratee = _.iteratee,
+        pass = false;
+
+    _.iteratee = identity;
+
+    try {
+      var actual = _(largeArray).slice(1).compact().value();
+      pass = lodashStable.isEqual(actual, compact(slice(largeArray, 1)));
+    } catch (e) {console.log(e);}
+
+    assert.ok(pass);
+    _.iteratee = iteratee;
+  });
+});