Phase 2: TEST_SCHEMA switching in test/install + schema-invariant assertions

[jnasbyupgrade]

Stacked on #27 (phase 1: test/install adoption). Adds the schema-targeting axis that #27 deliberately deferred.

What changed

• `test/install/load.sql` reads a new `count_nulls.test_schema` GUC (`TEST_SCHEMA` make var, same PGOPTIONS-propagation mechanism as before) to decide whether to `CREATE SCHEMA`/`SET search_path` before installing. Empty (default): no targeting at all. Non-empty: explicit target, including a name requiring SQL identifier quoting (`TEST_SCHEMA=Quoted`).
• `test/core/functions.sql`'s pgTAP assertions now pass an explicit, schema-free description to every call, instead of letting pgTAP's own auto-generated descriptions (which embed the real schema via `%I`) through. The SQL actually executed still goes through `ncs()` and is always correct regardless of where count_nulls landed - only the visible description text stops varying.
• Result: `test/expected/extension_tests.out` stays a single file both the empty and `Quoted` `TEST_SCHEMA` legs pass against. Verified directly: diffed a real `TEST_SCHEMA=Quoted` run against the empty-schema baseline - identical except one line.
• That one line is a genuine, correct behavioral difference, not an artifact: `test__shutdown__drop_all` drops the schema `TEST_SCHEMA` created, which only exists to drop when `TEST_SCHEMA` is non-empty. Handled via `pg_regress`'s native numbered-alternate mechanism - `test/expected/extension_tests_1.out`, captured from a real run (never hand-authored) - documented in `test/README.md`.
• Crossed the `test` CI job with `TEST_SCHEMA={"", Quoted}`.

Why this matters for what's next

This is the piece that makes crossing update/upgrade testing WITH schema scenarios (a later phase) free - no additional expected-output files needed there either, since the invariance holds regardless of how count_nulls got installed, not just for a fresh install.

Verification

`make verify-results` passes for both the default and `TEST_SCHEMA=Quoted` legs against this container's live PG17.

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