Add additional tests for help topic syntax

I don't think that is quite right:

     // Verify that if we remove all the translated text, whitespace, and
     // HTML tags, there is nothing left (that is, all text is translated).
     $text = preg_replace('|\{\% trans \%\}.*\{\% endtrans \%\}|sU', '', $body);
+    $result = locale_string_is_safe($text);
+    $this->assertTrue($result, 'Topic ' . $id . 'Twig file has all translatable strings exportable');

The line above the 2 lines you added removes all the translated text. So... it is opposite of what we want to test -- you are checking that the text that is *outside* the translated text is safe. Oops.

This is why we need a new test case to test the test. :)

Adding some more examples to issue summary of bad HTML syntax.

Here's a new patch for this, which I expect to fail... I need to upload it here and then fire up my dev machine so I can work on it and test as I go.

Here's a patch that passes locally. What it does:

a) Tests that the translated strings work in locale_string_is_safe().

b) Fixes the HTML syntax check so that it is more rigorous:
- Uses loadXML instead of loadHTML so that it validates the XML syntax (not against a DTD but at least that open/close tags match)
- Removes the translated strings and validates again so that it verifies we don't have situations like an opening tag outside and closing tag inside a single translated string

c) Adds some additional test cases to validate that the tests are able to catch invalid HTML

d) Adds a line to the test of the bad topics so it makes sure an exception/fail was actually encountered.

e) Hm.... just realized I could also validate that each translated string is separately valid HTML. So, there are 2 interdiffs here for the above and for this change.

f) [edit: added this] The additional tests in this patch found HTML syntax problems in 5 existing topics (fixes for those are included in the patch, because the tests failed locally without those fixes).

The test result showed a coding standards error in this one line:

catch (ExpectationFailedException|AssertionFailedError $e) {

Needs spaces before/after the |

Here's a new patch that does this. I edited the patch file directly so no interdiff, hope that is OK.

Well, since the time I made the previous patch and now, some new topics were committed that these tests found errors in. Here's a new patch and interdiff. I am not on my dev machine so I'm not sure the tests will pass, but I did fix two HTML validation problems in the migrate.overview topic.... let's see what the bot says. And hopefully get this back to RTBC and committed before we commit any more topics with errors!

Stray DL tag.

Typo in that last patch. This line wasn't supposed to be changed:

----
+--

Sheesh. Trying once more...

THANK YOU for committing this! It will indeed be very useful for preventing problems, as you can see by all the HTML syntax errors it caught that I had to put in the patch so the tests would pass. :)

Yes, it should be backported to 9.1, along with
#3047711: Convert file, image, media, media_library, responsive_image module hook_help() to topic(s)
#3073476: Convert migrate, migrate_drupal, migrate_drupal_multilingual, migrate_drupal_ui module hook_help() to topic(s)
The policy is apparently that patches on experimental modules get backported. I have a Slack thread about those other two issues; I'll add this to the thread.