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/dropWhile.js

@@ -0,0 +1,67 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { slice, LARGE_ARRAY_SIZE } from './utils.js';
+import dropWhile from '../dropWhile.js';
+
+describe('dropWhile', function() {
+  var array = [1, 2, 3, 4];
+
+  var objects = [
+    { 'a': 2, 'b': 2 },
+    { 'a': 1, 'b': 1 },
+    { 'a': 0, 'b': 0 }
+  ];
+
+  it('should drop elements while `predicate` returns truthy', function() {
+    var actual = dropWhile(array, function(n) {
+      return n < 3;
+    });
+
+    assert.deepStrictEqual(actual, [3, 4]);
+  });
+
+  it('should provide correct `predicate` arguments', function() {
+    var args;
+
+    dropWhile(array, function() {
+      args = slice.call(arguments);
+    });
+
+    assert.deepStrictEqual(args, [1, 0, array]);
+  });
+
+  it('should work with `_.matches` shorthands', function() {
+    assert.deepStrictEqual(dropWhile(objects, { 'b': 2 }), objects.slice(1));
+  });
+
+  it('should work with `_.matchesProperty` shorthands', function() {
+    assert.deepStrictEqual(dropWhile(objects, ['b', 2]), objects.slice(1));
+  });
+
+  it('should work with `_.property` shorthands', function() {
+    assert.deepStrictEqual(dropWhile(objects, 'b'), objects.slice(2));
+  });
+
+  it('should work in a lazy sequence', function() {
+    var array = lodashStable.range(1, LARGE_ARRAY_SIZE + 3),
+        predicate = function(n) { return n < 3; },
+        expected = dropWhile(array, predicate),
+        wrapped = _(array).dropWhile(predicate);
+
+    assert.deepEqual(wrapped.value(), expected);
+    assert.deepEqual(wrapped.reverse().value(), expected.slice().reverse());
+    assert.strictEqual(wrapped.last(), _.last(expected));
+  });
+
+  it('should work in a lazy sequence with `drop`', function() {
+    var array = lodashStable.range(1, LARGE_ARRAY_SIZE + 3);
+
+    var actual = _(array)
+      .dropWhile(function(n) { return n == 1; })
+      .drop()
+      .dropWhile(function(n) { return n == 3; })
+      .value();
+
+    assert.deepEqual(actual, array.slice(3));
+  });
+});