test: run ./xmodule/ tests with CMS settings (#33534)

Currently, ./xmodule/ unit tests are only run with LMS settings. However,
./common/ and ./xmodule/ are run twice: once with LMS settings and once with
CMS settings.

Just like ./common/ and ./openedx/, the unit tests in ./xmodule/ validate
behavior in both LMS and CMS. So, order to fully test ./xmodule/, we should to
run its tests with CMS settings too.

This will enable us to better validate certain LibraryContentBlocks behaviors
being touched by https://github.com/openedx/edx-platform/pull/33263 which can't
be expressed under LMS settings.

Also in this commit:

* refactor: rename the shards to be clear whether they're running under LMS or CMS
* docs: correct comments regarding conditions under which codejail's
   test_cant_do_something_forbidden is skipped.
* test: update a unit test which was using the now-deleted library_sourced block to use
   library_content block instead.

xmodule/tests/test_capa_block.py

@@ -29,6 +29,7 @@ from xblock.fields import ScopeIds
 from xblock.scorable import Score
 
 import xmodule
+from openedx.core.djangolib.testing.utils import skip_unless_lms
 from xmodule.capa import responsetypes
 from xmodule.capa.correctmap import CorrectMap
 from xmodule.capa.responsetypes import LoncapaProblemError, ResponseError, StudentInputError
@@ -182,6 +183,7 @@ if submission[0] == '':
 
 
 @ddt.ddt
+@skip_unless_lms
 class ProblemBlockTest(unittest.TestCase):  # lint-amnesty, pylint: disable=missing-class-docstring
 
     def setUp(self):
@@ -2898,6 +2900,7 @@ class ComplexEncoderTest(unittest.TestCase):  # lint-amnesty, pylint: disable=mi
         # ignore quotes
 
 
+@skip_unless_lms
 class ProblemCheckTrackingTest(unittest.TestCase):
     """
     Ensure correct tracking information is included in events emitted during problem checks.