Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/test/token/ERC721/ERC721.test.js

@@ -0,0 +1,15 @@
+const { shouldBehaveLikeERC721, shouldBehaveLikeERC721Metadata } = require('./ERC721.behavior');
+
+const ERC721 = artifacts.require('$ERC721');
+
+contract('ERC721', function (accounts) {
+  const name = 'Non Fungible Token';
+  const symbol = 'NFT';
+
+  beforeEach(async function () {
+    this.token = await ERC721.new(name, symbol);
+  });
+
+  shouldBehaveLikeERC721('ERC721', ...accounts);
+  shouldBehaveLikeERC721Metadata('ERC721', name, symbol, ...accounts);
+});