Revert "tests: context: disable if DEMAND_PAGING"

This reverts commit 79d7306.
The issue #31333 is fixed so this can be reverted to
enable tests/kernel/context to run with demand paging enabled.

Signed-off-by: Daniel Leung <[email protected]>

Author: dcpleung

tests/kernel/context/testcase.yaml

@@ -1,5 +1,3 @@
 tests:
-  kernel.context:
+  kernel.common:
     tags: kernel
-    # Bug is #31333
-    filter: not CONFIG_DEMAND_PAGING