Add deadlock test

Refs: #7
PR-URL: #18

test.js

@@ -1,6 +1,13 @@
 'use strict';
 
-const tests = ['simple', 'nested', 'steps', 'exclusive', 'thread-main'];
+const tests = [
+  'simple',
+  'nested',
+  'steps',
+  'exclusive',
+  'deadlock',
+  'thread-main',
+];
 
 (async () => {
   for (const name of tests) {

test/deadlock.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const assert = require('assert').strict;
+
+const { locks } = require('..');
+
+const TEST_TIMEOUT = 100;
+
+module.exports = async () =>
+  new Promise(resolve => {
+    let flag1 = false;
+    let flag2 = false;
+    let flag3 = false;
+    let flag4 = false;
+
+    (async () => {
+      await locks.request('C', async () => {
+        flag1 = true;
+        await locks.request('D', async () => {
+          flag2 = true;
+        });
+      });
+    })();
+
+    (async () => {
+      await locks.request('D', async () => {
+        flag3 = true;
+        await locks.request('C', async () => {
+          flag4 = true;
+        });
+      });
+    })();
+
+    (async () => {
+      setTimeout(() => {
+        assert.strictEqual(flag1, true);
+        assert.strictEqual(flag2, false);
+        assert.strictEqual(flag3, true);
+        assert.strictEqual(flag4, false);
+        resolve();
+      }, TEST_TIMEOUT);
+    })();
+  });