test: tests pin counts of registry members, table entries and source occurrences #2324

Is your feature request related to a problem? Please describe.
Under the no-counted-numbers ruling, count pins and their tests go (as in #1811 and #1820). These remain:

• test_tcp_mptcp_error_message_unit: source.count("f'{self.alias}: [OptNo") pinned exactly
• test_mh_flags_reserved_unit.test_every_site_is_covered: len(FRAMES) (the set equality beside it covers this)
• test_pyshark_unit.test_lookup_table_shapes: both table lengths, and the docstring counts
• test_const_enum_no_mint: ALL_105_REGISTRIES, test_register_actually_mints_on_every_one_of_the_105_registries, len(excluded) and its message
• test_const_str_payload_870_unit: test_every_one_of_the_40_registered_members_…, len(members) and checked pins
• test_pcapng_unit: len(TLSKeyLabel.__members__) beside the set equality; test_fields_misc: len(discovered) beside the set equality; test_tier_guard: len(gap.gates)
• test_tier_guard: the message "none of its five explicit TEST_PATHS lists" (and its "no longer reaches")

Describe the solution you'd like
Drop each pin where a set equality already covers it. Otherwise assert the property the count stood for, e.g. every [OptNo message has one separator, or the encapsulation table is larger than the filter-name one. Rename the counted names to say what they cover.

Additional context
These go red on a vendor update or an unrelated edit, for no defect.