Fix test infrastructure: testing.postgresql 1.3.0 incompatible with PostgreSQL 17 on macOS ARM

[myyong]

Problem

123 tests error with RuntimeError: *** failed to launch Postgresql *** when running the test suite on macOS aarch64 with PostgreSQL 17 (Homebrew).

The PostgreSQL log shows:

FATAL:  postmaster became multithreaded during startup

Root cause

testing.postgresql 1.3.0 (last updated 2019) starts PostgreSQL by running the postgres binary directly via subprocess.Popen. PostgreSQL 17 added a macOS-specific startup check (pthread_main_np()) that refuses to start if the process appears to have been launched from a multithreaded context. By the time unittest is running (which itself creates threads), every attempt to start a test PostgreSQL instance fails this check.

Environment where this occurs:

• macOS aarch64 (Apple Silicon)
• PostgreSQL 17.x (Homebrew)
• Python 3.12
• testing.postgresql 1.3.0

Affected tests

All 123 tests in test classes that inherit from RequiresDBTestCase with database_type = TestPostgres (the default). DuckDB-based tests are unaffected.

Fix options

1. Replace testing.postgresql with pytest-postgresql — actively maintained, has explicit PostgreSQL 17 support. Requires migrating from unittest to pytest.

2. Subclass testing.postgresql.Postgresql to override start() and launch the postgres process via os.fork() + os.execvp() directly from a single-threaded context (avoids the posix_spawn path that Python 3.12 uses on macOS).

3. Downgrade PostgreSQL to 16.x for local development (workaround only).

Notes

• The DuckDB test path (TestDuckDb) is unaffected and covers the same test surface for dialect-agnostic code.
• This issue pre-exists on the mssql branch and is not introduced by the main merge.