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/toString.test.js

@@ -0,0 +1,55 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+import { stubString, symbol } from './utils.js';
+import toString from '../toString.js';
+
+describe('toString', function() {
+  it('should treat nullish values as empty strings', function() {
+    var values = [, null, undefined],
+        expected = lodashStable.map(values, stubString);
+
+    var actual = lodashStable.map(values, function(value, index) {
+      return index ? toString(value) : toString();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should preserve the sign of `0`', function() {
+    var values = [-0, Object(-0), 0, Object(0)],
+        expected = ['-0', '-0', '0', '0'],
+        actual = lodashStable.map(values, toString);
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should preserve the sign of `0` in an array', function() {
+    var values = [-0, Object(-0), 0, Object(0)];
+    assert.deepStrictEqual(toString(values), '-0,-0,0,0');
+  });
+
+  it('should not error on symbols', function() {
+    if (Symbol) {
+      try {
+        assert.strictEqual(toString(symbol), 'Symbol(a)');
+      } catch (e) {
+        assert.ok(false, e.message);
+      }
+    }
+  });
+
+  it('should not error on an array of symbols', function() {
+    if (Symbol) {
+      try {
+        assert.strictEqual(toString([symbol]), 'Symbol(a)');
+      } catch (e) {
+        assert.ok(false, e.message);
+      }
+    }
+  });
+
+  it('should return the `toString` result of the wrapped value', function() {
+    var wrapped = _([1, 2, 3]);
+    assert.strictEqual(wrapped.toString(), '1,2,3');
+  });
+});