Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/test/token/ERC721/utils/ERC721Holder.test.js

@@ -0,0 +1,22 @@
+const { expect } = require('chai');
+
+const ERC721Holder = artifacts.require('ERC721Holder');
+const ERC721 = artifacts.require('$ERC721');
+
+contract('ERC721Holder', function (accounts) {
+  const [owner] = accounts;
+
+  const name = 'Non Fungible Token';
+  const symbol = 'NFT';
+  const tokenId = web3.utils.toBN(1);
+
+  it('receives an ERC721 token', async function () {
+    const token = await ERC721.new(name, symbol);
+    await token.$_mint(owner, tokenId);
+
+    const receiver = await ERC721Holder.new();
+    await token.safeTransferFrom(owner, receiver.address, tokenId, { from: owner });
+
+    expect(await token.ownerOf(tokenId)).to.be.equal(receiver.address);
+  });
+});