Reject custom test log lists whose logs lack the "test" marker

[robstradling]

Problem

A custom test log list (`strategy.testLogListFilename`) replaces the test TLS logs, but `LoadCustomTestLogList` only reassigns `TestTLSLogs` — it never touches the active/usable lists (which are frozen at package init from the embedded `ctloglists.CrtshV3Active`).

If a log is present in both the embedded active log list and the custom test file, it appears in `active_tls_logs.json` and `test_tls_logs.json`, because there is no de-duplication. That's the source of the "active endpoint emits custom logs" report. It's aggravated by `determineActiveTLSLogs` classifying a log as active whenever `log_type != "test"`, so a test log that omits the marker is treated as active.

Fix

Require every log and tiled log in a custom test log list to set `"log_type": "test"`. `validateLogList` now rejects any that don't, so `LoadCustomTestLogList` returns an error and ctsubmit fails fast at startup (via `lgr.Fatal`) rather than serving a non-test log as active/usable.

Tests

• Added `log_without_test_marker` and `tiled_log_without_test_marker` cases to `TestValidateLogList`.
• Updated the `writeCustomTestLogList` helper and existing valid cases to carry `log_type: "test"`.
• `go build ./...` and `go test ./...` pass.

Docs

• `docs/INSTALL.md` now states the `log_type: "test"` requirement.