Phase 1: adopt test/install, decoupling installation from the test suite

[jnasbyupgrade]

Stacked on #25 (pgxntool 2.3.0 bump). First of several phases adding update+upgrade (U&U) test coverage - see pgxntool/README.asc's `test/install` and "Update & Upgrade (U&U) Testing" sections, which this follows directly (and which, worth noting, cites this repo's own prior `test/sql/extension_tests.sql` at an old commit as a schema-targeting example, with an explicit caveat against copying its plpgsql-function-based assertion style - a caveat this redesign takes seriously).

What changed

• `test/install/load.sql`: installs count_nulls once, committed, before the main `test/sql/` schedule - replacing each test file's own per-test `CREATE EXTENSION` via `test/deps.sql`. Its own output isn't tracked (see `test/install/.gitignore`) - pg_regress resolves its expected/actual paths to the same file, so the diff is always self-identical regardless of content; correctness comes from the SQL itself failing loudly, not a textual comparison (matching cat_tools' own `test/install/load.sql`).
• Retired the old ad hoc pattern where each test file hardcoded its own literal schema name (`\set schema ...`) as a stand-in for schema-qualification coverage. `test/sql/simple.sql` (same assertions as `extension_tests.sql`, just a different hardcoded schema) is now a strict subset once there's one canonical install location - removed. `test/sql/sanity.sql`'s own bare `CREATE EXTENSION` would now conflict with test/install's already-committed install anyway, and its coverage was already a subset of `test__functionality` - removed too.
• `test/core/functions.sql`/`test/sql/extension_tests.sql`: dropped the now-undefined `:schema` psql variable references; simplified `test__check_ncs`/`test__shutdown__drop_all` to not assume a specific hardcoded target (a future schema-targeting phase will make these real assertions again).

Fresh-only, no schema targeting yet - deliberately scoped small. Schema-switching and update/upgrade mode selection are follow-up phases, stacked on this one.

Notable side effect

Since `test/install` runs in its own bare connection (not the in-suite session pgTAP's own `tap_setup.sql` runs in, which normally puts pgTAP's schema on `search_path` first), an untargeted `CREATE EXTENSION` now lands in `public` instead of pgTAP's own schema. This is actually a simplification: it's now the only possible landing location for "no schema specified," which incidentally forecloses a source of expected-output divergence between entry points (in-suite vs. an external bare-psql install) that a schema-targeting phase would otherwise have had to handle as a separate case.

Verification

`make test` / `make verify-results` pass against this container's live PG17 (2/2: `../install/load`, `extension_tests`).

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