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/startsWith.js

@@ -0,0 +1,47 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { MAX_SAFE_INTEGER, falsey, stubTrue } from './utils.js';
+import startsWith from '../startsWith.js';
+
+describe('startsWith', function() {
+  var string = 'abc';
+
+  it('should return `true` if a string starts with `target`', function() {
+    assert.strictEqual(startsWith(string, 'a'), true);
+  });
+
+  it('should return `false` if a string does not start with `target`', function() {
+    assert.strictEqual(startsWith(string, 'b'), false);
+  });
+
+  it('should work with a `position`', function() {
+    assert.strictEqual(startsWith(string, 'b', 1), true);
+  });
+
+  it('should work with `position` >= `length`', function() {
+    lodashStable.each([3, 5, MAX_SAFE_INTEGER, Infinity], function(position) {
+      assert.strictEqual(startsWith(string, 'a', position), false);
+    });
+  });
+
+  it('should treat falsey `position` values as `0`', function() {
+    var expected = lodashStable.map(falsey, stubTrue);
+
+    var actual = lodashStable.map(falsey, function(position) {
+      return startsWith(string, 'a', position);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should treat a negative `position` as `0`', function() {
+    lodashStable.each([-1, -3, -Infinity], function(position) {
+      assert.strictEqual(startsWith(string, 'a', position), true);
+      assert.strictEqual(startsWith(string, 'b', position), false);
+    });
+  });
+
+  it('should coerce `position` to an integer', function() {
+    assert.strictEqual(startsWith(string, 'bc', 1.2), true);
+  });
+});