LanguageNegotiation for kernel tests

Cool. I'm always interested in making Kernel tests easier, especially for multilingual stuff. 😉

A few docs nits, but one real point:

1. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   + * Identify language from $_SESSION['language'] and set a language url prefix.
   ...
   + * Use case: in kernel tests we cannot rely on the url prefix to change
   ...
   + * be able to test links with language url prefixes.
   

   s/url/URL/

2. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   + * getLangcode() is taken from LanguageNegotiationUrl and simplified.
   

   Can we add a @see pointing to exactly what class we're talking about here?

3. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   + * processOutbound() is taken from LanguageNegotiationSession and simplified.
   

   Ditto here.

4. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   + *   description = @Translation("Language from a session parameter. Outbound paths get prefix."),
   

   These aren't complete sentences. 😉 How about:

   "Gets the language from a session parameter. Outbound paths get a language prefix."

   or something?

5. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   +class LanguageNegotiationTestSessionAndUrl extends LanguageNegotiationMethodBase implements OutboundPathProcessorInterface {
   

   I don't think we'd commit this change to core without anything using it. Can we modify/add some Kernel tests somewhere to show off why this change makes life better?

6. +++ b/core/modules/language/tests/language_test/src/Plugin/LanguageNegotiation/LanguageNegotiationTestSessionAndUrl.php
   @@ -0,0 +1,71 @@
   +   * The language negotiation method id.
   

   s/id/ID/

NW mostly for point 5, but we might as well fix the others while we're at it...

Thanks!
-Derek

Admittedly I rerolled this when I was getting started. But regards to point 5 I don't have an actual answer so moving to PNMI as this may actually not be needed? Happily to be wrong.

If no one can see the use for this in core, maybe free to close out.