Minor improvements to "Hello world" (#1234)

* Adds some tests to ensure hardware outputs make sense. The most
important checks that the graph in [Step 4. Analyze the
results](https://docs.quantum.ibm.com/start/hello-world#step-4-analyze-the-results)
looks correct, as the current one doesn't match the description.
(compare to [new
graph](https://qiskit-docs-preview-pr-1234.1799mxdls7qz.us-south.codeengine.appdomain.cloud/start/hello-world#step-4-analyze-the-results)).
* Also makes some minor tweaks I noticed while looking for good places
for tests.
* Re-run notebook