Centralize Postgres test fixture in internal/test

Each package that exercises the database against a real Postgres
carried its own copy of the connection bootstrap and schema setup.
Those copies had already drifted: some keyed off PROBO_TEST_PG_ADDR
with hardcoded defaults, others off PROBO_TEST_PG_URL, and the
agentrun/coredata suites hand-applied individual agent_runs
migrations to ensure the table existed.

Introduce a single test.PGClient helper that parses PROBO_TEST_PG_URL
(falling back to the local compose database), runs the full coredata
migration set once per process, and skips when no database is
reachable so make test stays a pure unit-test run. Migrate the
agentrun, coredata, cookiebanner, iam, and thirdparty suites onto it
and delete the duplicated helpers so the bootstrap can no longer
diverge.

Signed-off-by: Bryan Frimin <bryan@probo.com>

pkg/coredata/pg_checkpointer_test.go

@@ -20,6 +20,7 @@ import (
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"go.probo.inc/probo/internal/test"
 	"go.probo.inc/probo/pkg/agent"
 	"go.probo.inc/probo/pkg/coredata"
 	"go.probo.inc/probo/pkg/gid"
@@ -29,7 +30,7 @@ import (
 func TestPGCheckpointer(t *testing.T) {
 	t.Parallel()
 
-	client := pgClient(t)
+	client := test.PGClient(t)
 	store := coredata.NewPGCheckpointer(client)
 
 	t.Run(