test(17-06): read -short through testing.Short in the Postgres harnesses

go test passes -test.short=true, so scanning os.Args for -test.short
never matched and every -short run started a Postgres container. The
three TestMains now parse the flags and ask testing.Short.

updates/postgres_test.go

@@ -3,6 +3,7 @@ package updates
 import (
 	"context"
 	"database/sql"
+	"flag"
 	"fmt"
 	"net/url"
 	"os"
@@ -43,13 +44,11 @@ func TestMain(m *testing.M) {
 	os.Exit(code)
 }
 
+// testShort reports -short. go test passes -test.short=true, so the flags
+// are parsed and read through testing.Short, never by scanning os.Args.
 func testShort() bool {
-	for _, a := range os.Args {
-		if a == "-test.short" {
-			return true
-		}
-	}
-	return false
+	flag.Parse()
+	return testing.Short()
 }
 
 func startJournalPostgres(ctx context.Context) error {