test: test names, constants and labels still describe a change, not the behaviour #2325

Is your feature request related to a problem? Please describe.
This continues #2227. These names say what changed rather than what they pin:

• the ci: divide and simplify the workflows into scripts, shared heads and smaller files #1967 parity tests and test_ktlo_legs_stay_green_1672: FORMER_*, TestTheFormerShellAgrees, former(), test_no_step_holds_the_former_shell, test_the_former_release_copy_…, "former shell" skip messages
• test_layer_placement_unit: …_unaffected_by_the_move, …_left_the_link_subpackage, …_did_not_move_with_the_modules, …_leaves_links_registry_behind_…, …_stops_at_ipv4_as_before, message "'link' must no longer terminate at OSPF"
• const: GetNoLongerMintsTests, RegisterAlreadyRegisteredNowRaises…, ExtensionHeaderBitEmuRemovedTests, …_the_code_bit_emu_used_to_occupy, test_tcp_flags_gains_the_protocol_it_never_had, test_transport_protocol_dropped_its_flag_base
• test_previously_shadowed_ranges_are_reachable, test_default_is_a_new_capability_not_exercised_before, test_registry_currently_matches_pinned_target, test_dev_carries_what_all_used_to_for_the_toolchain, test_old_call_sites_keep_catching, test_old_algorithm_would_have_written_inside_vendor
• *_as_before / *_as_it_was: test_names_and_values_resolve_as_before, test_an_unforced_bump_not_pushed_is_as_before, test_what_it_cannot_read_publishes_as_before, test_without_shards_pytest_runs_once_as_before, test_the_pull_request_comment_is_as_it_was, test_a_parsed_segment_is_reported_as_before
• labels: 'pre-#818 shape'/'post-#818 shape' (REPR_PERCENT_WALK_FIXTURES), 'as before #1674' (test_vendor_comment_only_no_bump_1674)

Describe the solution you'd like
Rename each to the behaviour it pins, e.g. REFERENCE_PLAN and TestTheStepMatchesTheReferenceShell. Update docstrings, comments and any guard that cites them.

Additional context
Search: grep -rnE "(def|class) \w*(former|Former|as_before|as_it_was|used_to|NoLonger|Now[A-Z]|previously|currently|Removed[A-Z]|_gains_|_dropped_its|new_capability)" tests/; not every hit is change-relative. Left out, as #2255 kept them: "no longer …" failure messages that name the regression, and _still_ control cases.