Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/test/token/ERC20/extensions/ERC20Burnable.test.js

@@ -0,0 +1,20 @@
+const { BN } = require('@openzeppelin/test-helpers');
+
+const { shouldBehaveLikeERC20Burnable } = require('./ERC20Burnable.behavior');
+const ERC20Burnable = artifacts.require('$ERC20Burnable');
+
+contract('ERC20Burnable', function (accounts) {
+  const [owner, ...otherAccounts] = accounts;
+
+  const initialBalance = new BN(1000);
+
+  const name = 'My Token';
+  const symbol = 'MTKN';
+
+  beforeEach(async function () {
+    this.token = await ERC20Burnable.new(name, symbol, { from: owner });
+    await this.token.$_mint(owner, initialBalance);
+  });
+
+  shouldBehaveLikeERC20Burnable(owner, initialBalance, otherAccounts);
+});