Add timestamp based governor with EIP-6372 and EIP-5805 (#3934)

Co-authored-by: Francisco Giordano <fg@frang.io>
Co-authored-by: Ernesto García <ernestognw@gmail.com>
Co-authored-by: Francisco <frangio.1@gmail.com>

test/helpers/time.js

@@ -0,0 +1,16 @@
+const { time } = require('@openzeppelin/test-helpers');
+
+module.exports = {
+  clock: {
+    blocknumber: () => web3.eth.getBlock('latest').then(block => block.number),
+    timestamp: () => web3.eth.getBlock('latest').then(block => block.timestamp),
+  },
+  clockFromReceipt: {
+    blocknumber: receipt => Promise.resolve(receipt.blockNumber),
+    timestamp: receipt => web3.eth.getBlock(receipt.blockNumber).then(block => block.timestamp),
+  },
+  forward: {
+    blocknumber: time.advanceBlockTo,
+    timestamp: time.increaseTo,
+  },
+};