logictest: support block-level skipif/onlyif for subtests

Currently `skipif`/`onlyif` applies only to the immediately following directive. To skip an entire subtest, you must repeat the same `skipif` before every `statement` or `query`, e.g.:

```
subtest enable_disable_trigger

skipif config local-mixed-25.4 local-mixed-26.1
statement ok
CREATE TABLE t (a INT);

skipif config local-mixed-25.4 local-mixed-26.1
statement ok
CREATE FUNCTION f() ...

# ... repeated 30+ more times
```

This is a real maintenance burden. Across the logictest suite, 35% of all `skipif`/`onlyif` directives (645 of 1,861) are part of 3+ consecutive identical repetitions spanning 53 files. The worst cases are `triggers` (69 consecutive identical `skipif` lines in one subtest) and `statement_hint_builtins` (63).

Proposal: allow `skipif`/`onlyif` immediately after a `subtest` directive to apply to the entire subtest block, so the above becomes:

```
subtest enable_disable_trigger
skipif config local-mixed-25.4 local-mixed-26.1

statement ok
CREATE TABLE t (a INT);

statement ok
CREATE FUNCTION f() ...
```

Jira issue: CRDB-64160

Start by locating the logictest handling for subtest, skipif, and onlyif directives, then inspect how statement and query entries are grouped into subtest blocks. Implement the proposed block-level behavior and verify that repeated directives are no longer required while existing directive behavior remains unchanged.

Written by the indexing model from the issue text.