Omit block for test failure details in annotation when there's none

[mokagio]

Not all test nodes failures in the JUnit report have a value.

E.g. from the failures in this build:

<testcase classname="StatsTests" name="testInsightsStatsLoadProperly()" time="131.52018702030182">
  <failure message="failed: caught error: "timedOut" (#CharacterRangeLen=0)"> </failure>
</testase>

In such cases, the failure annotation looks a bit odd:

The changes in this PR make the annotation look like this:

---

• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[mokagio]

I could be convinced that the <pre> makes sense only when there are no details.

In case both failure and details have a value, we'd get something like:

---

failure name

details details
details

---

That might not be the best to look at 🤔

Then again, we don't have to look at that info for long 😅

[crazytonyli]

Nitpick: I find three usages of # here a bit confusing. Not sure if this suggestion would make it better or worse 😄

Suggested change

	opts.on('--slack CHANNEL_NAME', 'The name of the Slack channel to post the failure summary to') { |v| slack_channel = "##{v.delete_prefix('#')}" }
	opts.on('--slack CHANNEL_NAME', 'The name of the Slack channel to post the failure summary to') { |v| slack_channel = v.delete_prefix('#').prepend('#') }

[mokagio]

We've been wondering with how to best test these changes and other changes. See internal ref p1685981991334859-slack-C02KLTL3MKM

It's crucial for the long term health of this project that we find an answer. However, I think the scope is broader than this PR and therefore I'm going to merge in the meantime to roll out the improvement.

On that note, I had a play around and have a proof of concept for using RSpec to test part of the code edited here: #64