Warn about experimental modules on their help pages

Working on tests.

Attached adds a test and also fixes a sloppy copy. The test module itself is added to system since it will be useful for other tests (e.g. #2560597: Mark Migrate* modules as Experimental itself did not add any test coverage).

I have some concern that we're making the "Core (Experimental)" package name very magical (in the original issue and again here). It might be better to add a boolean flag to the module info? That would not really have been a good choice in #2560597: Mark Migrate* modules as Experimental right before RC, but we could do it in a minor. Out of scope for this issue, but could be a followup.

Hm, assuming the test-only patch passed because of #2657482: Test bot doesn't apply the patch; we'll see what the next run reports.

Seems a shame to make install Drupal just to make one assertion. Couldn't this be part of Drupal\help\Tests\HelpTest::testHelp()?

No, I looked into that. HelpTest installs a load of crap, including standard and like all the things. Plus then we would need to add the experimental module to be installed in that test as well. Separate environment needed for the test, so separate installation makes sense.

Super minor nit... missing a docblock...

We have, or had, a standard that setUp() methods do not have docblocks. Has this changed? If it has, core is not compliant with the changed standard.

fail: [Other] Line 131 of core/modules/system/src/Tests/Module/InstallUninstallTest.php:
'Test Experimental Module module' is on the help page for experimental_module_test

fail: [Other] Line 131 of core/modules/system/src/Tests/Module/InstallUninstallTest.php:
Correct online documentation link is in the help page for experimental_module_test

Oh FFS. :P

Re: #6 point 2, I forgot that #338403: Use {@inheritdoc} on all class methods (including tests) got reopened and the policy changed. :)

Attached adds the {@inheritdoc} per #11, properly marks the experimental module as hidden, and makes it conform to the expected help documentation standards. I also made the version number match #2656994: Experimental modules should have their own version numbers. (Naturally, since we are testing the "Core (Experimental)" package, it can't be in the "Testing" package.)

Per #12, adding another assertion for a non-experimental module. We can't use help_test.module because that module is not actually for testing help -- it's for testing empty help. :P It should maybe be empty_help_test.module. But that's not in scope here, so I just added a separate test module.

I looked again at HelpTest and it really did not seem like a good fit in this case. There are many times when adding an assertion to an existing test is the best thing to do, but I don't think this is one of them.

Also adding a missing typehint to the first test module.

+++ b/core/modules/help/src/Tests/ExperimentalHelpTest.php
@@ -49,6 +49,10 @@ public function testExperimentalHelp() {
+    // Regular modules should not display the message.
+    $this->drupalGet('admin/help/help_page_test');
+    $this->assertNoText('This module is experimental.');

This set of assertions isn't strong enough to avoid a false positive (or is it false negative?); it could be a 403/404/etc. I'll add assertions.

Fixed in attached.

+++ b/core/modules/help/src/Tests/ExperimentalHelpTest.php
@@ -0,0 +1,62 @@
+   * overview page.

Leftover line from a copy/paste. Removed in attached. Leaving at RTBC since this is trivial/docs.