Add test for TLA DFS invariant

This behavior was introduced after the Top-Level Await proposal reached
stage 3: tc39/proposal-top-level-await#159

test/language/module-code/top-level-await/dfs-invariant-async_FIXTURE.js

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+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+await 0;
+globalThis.test262 = 'async';

test/language/module-code/top-level-await/dfs-invariant-direct-1_FIXTURE.js

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+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+import './dfs-invariant-async_FIXTURE.js';
+globalThis.test262 += ':direct-1';

test/language/module-code/top-level-await/dfs-invariant-direct-2_FIXTURE.js

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+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+import './dfs-invariant-async_FIXTURE.js';
+globalThis.test262 += ':direct-2';

test/language/module-code/top-level-await/dfs-invariant-indirect_FIXTURE.js

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+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+import './dfs-invariant-direct-1_FIXTURE.js';
+globalThis.test262 += ':indirect';

test/language/module-code/top-level-await/dfs-invariant.js

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+// Copyright (C) 2021 the V8 project authors. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+description: Parent completion orderings match the synchronous module behavior
+info: |
+  6.2.4 AsyncModuleExecutionFulfilled ( module )
+
+  [...]
+  5. Let _execList_ be a new empty List.
+  6. Perform ! GatherAsyncParentCompletions(_module_, _execList_).
+  7. Let _sortedExecList_ be a List of elements that are the elements of
+     _execList_, in the order in which they had their [[AsyncEvaluating]]
+     fields set to *true* in InnerModuleEvaluation.
+  8. Assert: All elements of _sortedExecList_ have their [[AsyncEvaluating]]
+     field set to *true*, [[PendingAsyncDependencies]] field set to 0 and
+     [[EvaluationError]] field set to *undefined*.
+  [...]
+
+  Dependency graph for this test:
+
+                             dfs-invariant.js
+  .-----------------------------------+-------.
+  |                                   |       v
+  |                                   |       dfs-invariant-indirect_FIXTURE.js
+  |                               .---|----------------------'
+  v                               v   v
+  dfs-invariant-direct-1_FIXTURE.js   dfs-invariant-direct-2_FIXTURE.js
+            '--------.                            .--------'
+                     v                            v
+                     dfs-invariant-async_FIXTURE.js
+esid: sec-moduleevaluation
+flags: [module]
+features: [top-level-await, globalThis]
+---*/
+
+import './dfs-invariant-direct-1_FIXTURE.js';
+import './dfs-invariant-direct-2_FIXTURE.js';
+import './dfs-invariant-indirect_FIXTURE.js';
+
+assert.sameValue(globalThis.test262, 'async:direct-1:direct-2:indirect');