Split SQL tests into core, integration, and performance suites

[seanwevans]

Motivation

• Reduce feedback time by keeping deterministic, fast SQL tests as the default suite (test-core).
• Move slower or non-deterministic checks (HTTPS transport and GC perf) to opt-in suites so they do not slow normal development.
• Make running slower suites explicit via environment flags to avoid accidental runs (RUN_INTEGRATION, RUN_PERF).
• Ensure CI runs fast core tests on every change and runs slower suites on a scheduled or manual cadence.

Description

• Reworked the makefile to introduce CORE_TESTS, INTEGRATION_TESTS, and PERFORMANCE_TESTS, and kept a backward-compatible TESTS aggregate.
• Added Make targets test-core, test-integration, test-performance, and test-all, and made the default test target an alias for test-core.
• Gated slow suites so test-integration requires RUN_INTEGRATION=1 and test-performance requires RUN_PERF=1, and each target prints a skip message when the flag is not set.
• Updated .github/workflows/ci.yml to run test-core on push/pull_request and to run integration/performance suites only on schedule or manual dispatch, and expanded README.md with suite descriptions, expected runtimes, prerequisites, and example commands (e.g. make test-core, RUN_INTEGRATION=1 make test-integration).

Testing

• Attempted to validate the new targets locally with make test-integration && make test-performance && make -n test-core to ensure the gating and default alias behave as intended.
• The local run could not complete because PGXS is not available in this environment (/usr/lib/postgresql/.../pgxs.mk: No such file or directory), so the automated test commands did not execute here.
• CI is configured to run test-core on push/PR and will run slow suites on schedule or when manually dispatched, where those jobs will exercise the opt-in behavior and environment flags.

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Codex Task