Check RPC after geth startup to reduce flakiness

This is meant to avoid the following errors that happens across most
(all?) e2e tests from time to time:
```
 Error: Connection refused or URL couldn't be resolved: http://localhost:8545
  at XMLHttpRequest.request.onreadystatechange (/home/circleci/app/node_modules/web3-providers/dist/web3-providers.cjs.js:759:22)
  at XMLHttpRequestEventTarget.dispatchEvent (/home/circleci/app/node_modules/xhr2-cookies/xml-http-request-event-target.ts:44:13)
  at XMLHttpRequest._setReadyState (/home/circleci/app/node_modules/xhr2-cookies/xml-http-request.ts:219:8)
  at XMLHttpRequest._onHttpRequestError (/home/circleci/app/node_modules/xhr2-cookies/xml-http-request.ts:379:8)
  at ClientRequest.<anonymous> (/home/circleci/app/node_modules/xhr2-cookies/xml-http-request.ts:266:37)
  at Socket.socketErrorListener (_http_client.js:406:9)
  at emitErrorNT (internal/streams/destroy.js:92:8)
  at emitErrorAndCloseNT (internal/streams/destroy.js:60:3)
  at processTicksAndRejections (internal/process/task_queues.js:80:21)
```

I am not sure this actually works as intended, but I could not reproduce
the problem after this change, anymore. So it is worth testing it out,
as we can easily revert the change or improve the approach.

packages/celotool/src/lib/geth.ts

@@ -1274,8 +1274,26 @@ export async function startGeth(
     }
   }
 
-  // Geth startup isn't fully done even when the port is open, so give it another second
-  await sleep(1000)
+  // Geth startup isn't fully done even when the port is open, so check until it responds
+  const maxTries = 5
+  let tries = 0
+  while (tries < maxTries) {
+    tries++
+    let block = null
+    try {
+      block = await new Web3('http://localhost:8545').eth.getBlock('latest')
+    } catch (e) {
+      console.log(`Failed to fetch test block: ${e}`)
+    }
+    if (block) {
+      break
+    }
+    console.log('Could not fetch test block. Wait one second, then retry.')
+    await sleep(1000)
+  }
+  if (tries === maxTries) {
+    throw new Error(`Geth did not start within ${tries} seconds`)
+  }
 
   console.log(
     `${instance.name}: running.`,