Rename ctests with chimaera_ prefix to cr_

[lukemartinlogan]

Description

Three registered ctests still carry the legacy `chimaera_` prefix instead
of the current `cr_` (Clio Runtime) naming used elsewhere in the suite.
This is a pure rename — no behavior changes.

Renames

Old name	New name
`chimaera_pre_test_cleanup`	`cr_pre_test_cleanup`
`chimaera_post_test_cleanup`	`cr_post_test_cleanup`
`cr_chimaera_unit_tests`	`cr_unit_tests`

Scope

• `CMakeLists.txt` (top-level): the two cleanup-fixture `add_test` blocks
  (POSIX and Windows variants) and the fixture-attachment exclusion check
  at `_chimaera_require_cleanup_fixture_recursive`.
• `context-runtime/test/unit/CMakeLists.txt`: the
  `cr_chimaera_unit_tests` `add_test` block and one downstream
  dependency reference.

The internal fixture identifier `chimaera_test_cleanup_fixture` (a CMake
property value, not a ctest name) is left alone to keep the diff minimal.

Acceptance Criteria

• `ctest -N` shows the three new names and zero results for
  `ctest -N | grep -i chimaera`.
• Cleanup fixture still chains: a normal ctest run still invokes
  pre-cleanup before the suite and post-cleanup after.