GSN compatibility (#1880)

* switch to using Context internally

* add context import

* Add smoke test to make sure enabling GSN support works

* Update test/GSN/ERC721GSNRecipientMock.test.js

Co-Authored-By: Francisco Giordano <frangio.1@gmail.com>

* Upgrade truffle

* add missing awaits

* Revert "Upgrade truffle"

This reverts commit f9b0ba9019.

test/GSN/ERC721GSNRecipientMock.test.js

@@ -0,0 +1,46 @@
+const { constants, expectEvent } = require('openzeppelin-test-helpers');
+const { ZERO_ADDRESS } = constants;
+const gsn = require('@openzeppelin/gsn-helpers');
+const { fixSignature } = require('../helpers/sign');
+const { utils: { toBN } } = require('web3');
+
+const ERC721GSNRecipientMock = artifacts.require('ERC721GSNRecipientMock');
+
+contract('ERC721GSNRecipient (integration)', function ([_, signer, sender]) {
+  const tokenId = '42';
+
+  beforeEach(async function () {
+    this.token = await ERC721GSNRecipientMock.new(signer);
+  });
+
+  async function testMintToken (token, from, tokenId, options = {}) {
+    const { tx } = await token.mint(tokenId, { from, ...options });
+    await expectEvent.inTransaction(tx, ERC721GSNRecipientMock, 'Transfer', { from: ZERO_ADDRESS, to: from, tokenId });
+  }
+
+  context('when called directly', function () {
+    it('sender can mint tokens', async function () {
+      await testMintToken(this.token, sender, tokenId);
+    });
+  });
+
+  context('when relay-called', function () {
+    beforeEach(async function () {
+      await gsn.fundRecipient(web3, { recipient: this.token.address });
+    });
+
+    it('sender can mint tokens', async function () {
+      const approveFunction = async (data) =>
+        fixSignature(
+          await web3.eth.sign(
+            web3.utils.soliditySha3(
+              // eslint-disable-next-line max-len
+              data.relayerAddress, data.from, data.encodedFunctionCall, toBN(data.txFee), toBN(data.gasPrice), toBN(data.gas), toBN(data.nonce), data.relayHubAddress, this.token.address
+            ), signer
+          )
+        );
+
+      await testMintToken(this.token, sender, tokenId, { useGSN: true, approveFunction });
+    });
+  });
+});