test: remove useless tests

[spydon]

What

Removes tests that assert nothing meaningful, duplicate other tests, or only exercise behavior that Dart itself guarantees. No production code or genuinely useful coverage is touched.

406 deletions across 5 test files.

Removed

gotrue/test/src/constants_test.dart

• constants are consistent — re-asserts the same constants in different units (Dart Duration arithmetic).
• v20240101 is UTC timestamp — asserts DateTime.parse('...Z').isUtc, a language guarantee.
• The enum names match expected values test for OtpType — asserts enum.name, but OtpType serializes to the wire via .snakeCase, not .name, so it only exercised Dart's .name. (The OtpChannel and SignOutScope equivalents are kept and reworded as wire-value tests, since those enums are serialized via .name.)
• The Enum consistency group — toString, ==, and set/map usage of enums are all Dart guarantees.
• The Documentation and comments group — re-tests enum.name (duplicates the above).
• The Real-world usage scenarios group — tests a local switch helper, arithmetic on hand-built lists, and fromString round-trips already covered by the kept GenerateLinkTypeExtended tests.

Kept the constant value checks (they guard wire-protocol contracts), the jsName/fromString/snakeCase mapping tests, the custom ApiVersions.name/timestamp getters, and the enum-membership (values/contains) checks.

gotrue/test/src/types/auth_exception_test.dart

• all exception types implement Exception and all exception types extend AuthException — assert declared inheritance, fully redundant with the per-subclass extends AuthException tests.

postgrest/test/transforms_test.dart

• The maxAffected group (5 returnsNormally smoke tests) — redundant with the maxAffected integration group right below it, which actually asserts the resulting Prefer headers.

supabase/test/client_test.dart

• Schema Support, RPC Support, Client Options groups — only assert that statically-typed methods return their declared type (tautological isA); real coverage lives in mock_test.dart and postgrest/test/basic_test.dart.
• Query Schema group — exact duplicate of Schema Support.
• should properly dispose all resources — no assertions; dispose is covered with real assertions by the kept Shared YAJsonIsolate group.

supabase_flutter/test/supabase_flutter_test.dart

• The five single-method EmptyLocalStorage tests — fully subsumed by the kept all methods work together in a typical flow test, which asserts the same contract end to end.

Test plan

• dart analyze passes on all five files (no unused imports left behind).
• Ran the affected suites that don't require a live backend: gotrue constants_test/auth_exception_test, supabase client_test, and supabase_flutter supabase_flutter_test all pass. postgrest/transforms_test is an integration suite needing a local PostgREST server, untouched logic.