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/isFunction.js

@@ -0,0 +1,83 @@
+import assert from 'assert';
+import lodashStable from 'lodash';
+
+import {
+  slice,
+  asyncFunc,
+  genFunc,
+  arrayViews,
+  objToString,
+  funcTag,
+  falsey,
+  stubFalse,
+  args,
+  symbol,
+  document,
+  realm,
+} from './utils.js';
+
+import isFunction from '../isFunction.js';
+
+describe('isFunction', function() {
+  it('should return `true` for functions', function() {
+    assert.strictEqual(isFunction(_), true);
+    assert.strictEqual(isFunction(slice), true);
+  });
+
+  it('should return `true` for async functions', function() {
+    assert.strictEqual(isFunction(asyncFunc), typeof asyncFunc == 'function');
+  });
+
+  it('should return `true` for generator functions', function() {
+    assert.strictEqual(isFunction(genFunc), typeof genFunc == 'function');
+  });
+
+  it('should return `true` for the `Proxy` constructor', function() {
+    if (Proxy) {
+      assert.strictEqual(isFunction(Proxy), true);
+    }
+  });
+
+  it('should return `true` for array view constructors', function() {
+    var expected = lodashStable.map(arrayViews, function(type) {
+      return objToString.call(root[type]) == funcTag;
+    });
+
+    var actual = lodashStable.map(arrayViews, function(type) {
+      return isFunction(root[type]);
+    });
+
+    assert.deepStrictEqual(actual, expected);
+  });
+
+  it('should return `false` for non-functions', function() {
+    var expected = lodashStable.map(falsey, stubFalse);
+
+    var actual = lodashStable.map(falsey, function(value, index) {
+      return index ? isFunction(value) : isFunction();
+    });
+
+    assert.deepStrictEqual(actual, expected);
+
+    assert.strictEqual(isFunction(args), false);
+    assert.strictEqual(isFunction([1, 2, 3]), false);
+    assert.strictEqual(isFunction(true), false);
+    assert.strictEqual(isFunction(new Date), false);
+    assert.strictEqual(isFunction(new Error), false);
+    assert.strictEqual(isFunction({ 'a': 1 }), false);
+    assert.strictEqual(isFunction(1), false);
+    assert.strictEqual(isFunction(/x/), false);
+    assert.strictEqual(isFunction('a'), false);
+    assert.strictEqual(isFunction(symbol), false);
+
+    if (document) {
+      assert.strictEqual(isFunction(document.getElementsByTagName('body')), false);
+    }
+  });
+
+  it('should work with a function from another realm', function() {
+    if (realm.function) {
+      assert.strictEqual(isFunction(realm.function), true);
+    }
+  });
+});