test: honour -short in Docker-backed TestMain and cabana list-service fixture

TestMain scanned os.Args for a literal -test.short, but go test passes
-test.short=true, so beachcomber, conga, lagoon and lighthouse started a
Postgres container even under -short. Parse the flags and use
testing.Short() instead; skip cabana's newListService fixture under -short
like adminGorm already does.

modules/lighthouse/postgres_test.go

@@ -3,6 +3,7 @@ package lighthouse
 import (
 	"context"
 	"database/sql"
+	"flag"
 	"fmt"
 	"net/url"
 	"os"
@@ -42,13 +43,14 @@ func TestMain(m *testing.M) {
 	os.Exit(code)
 }
 
+// testShort reports whether -short was passed. TestMain runs before the
+// testing flags are parsed, and go test passes the flag as -test.short=true,
+// so parse the flags here instead of scanning os.Args.
 func testShort() bool {
-	for _, a := range os.Args {
-		if a == "-test.short" {
-			return true
-		}
+	if !flag.Parsed() {
+		flag.Parse()
 	}
-	return false
+	return testing.Short()
 }
 
 func startLighthousePostgres(ctx context.Context) error {