Rename test_... classes to avoid the naming scheme reserved by pytest

[tobiasdiez]

Similar to #33549, but now for classes and not functions.

From #33549 comment:13

CC: @mkoeppe @soehms @tscrim

Component: doctest framework

Author: Tobias Diez

Branch/Commit: public/tests/pytest_rename_test_classes @ 969ef9d

Issue created by migration from https://trac.sagemath.org/ticket/33561

[mkoeppe]

Description changed:

--- 
+++ 
@@ -1 +1,3 @@
 Similar to #33549, but now for classes and not functions.
+
+From [#33549 comment:13](https://github.com/sagemath/sage/issues/33549#comment:13)

[tobiasdiez]

New commits:

11a54cf	Rename category TestObjects
9efaf59	Rename test classes in explain_pickle
e61cdbb	Rename test classes in nested_class_test
1f3b652	Rename test classes in backend_test
aa78065	Rename class in dynamic_class
d4481b2	Rename test class in arithgroup
4cbfec1	Rename test class in moodsym
7c6a0e2	Fix tests

[tobiasdiez]

Commit: 7c6a0e2

[tobiasdiez]

Branch: public/tests/pytest_rename_test_classes

[sagetrac-git]

Changed commit from 7c6a0e2 to 969ef9d

[sagetrac-git]

Branch pushed to git repo; I updated commit sha1. New commits:

969ef9d

Merge remote-tracking branch 'origin/develop' into public/tests/pytest_rename_test_classes