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/spread.js

@@ -0,0 +1,58 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice, _, stubTrue, falsey } from './utils.js';
+
+describe('spread', function() {
+  function fn(a, b, c) {
+    return slice.call(arguments);
+  }
+
+  it('should spread arguments to `func`', function() {
+    var spread = _.spread(fn),
+        expected = [1, 2];
+
+    assert.deepStrictEqual(spread([1, 2]), expected);
+    assert.deepStrictEqual(spread([1, 2], 3), expected);
+  });
+
+  it('should accept a falsey `array`', function() {
+    var spread = _.spread(stubTrue),
+        expected = lodashStable.map(falsey, stubTrue);
+
+    var actual = lodashStable.map(falsey, function(array, index) {
+      try {
+        return index ? spread(array) : spread();
+      } catch (e) {}
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should work with `start`', function() {
+    var spread = _.spread(fn, 1),
+        expected = [1, 2, 3];
+
+    assert.deepStrictEqual(spread(1, [2, 3]), expected);
+    assert.deepStrictEqual(spread(1, [2, 3], 4), expected);
+  });
+
+  it('should treat `start` as `0` for negative or `NaN` values', function() {
+    var values = [-1, NaN, 'a'],
+        expected = lodashStable.map(values, lodashStable.constant([1, 2]));
+
+    var actual = lodashStable.map(values, function(value) {
+      var spread = _.spread(fn, value);
+      return spread([1, 2]);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should coerce `start` to an integer', function() {
+    var spread = _.spread(fn, 1.6),
+        expected = [1, 2, 3];
+
+    assert.deepStrictEqual(spread(1, [2, 3]), expected);
+    assert.deepStrictEqual(spread(1, [2, 3], 4), expected);
+  });
+});