Move the test-only helpers out of the shipped db package (closes #248)

The server binary carried exported names that only tests call, including
raw SQL access and a way to weaken the password hash.

Delete testing.go, GetDB and SetBcryptCost from the shipped code, so
passwords always hash at bcrypt.DefaultCost. Move GetDB, GetOrCreateChannel,
JoinChannel and HasChannelInvite into export_test.go. Delete
GetClientHostInfo, whose remaining check GetLatestClientForSession covers.

Tests in other packages now build the database with db.New and run raw SQL
through dbtest.Exec.

Model: opus-5-5

internal/handlers/api_test.go

@@ -23,7 +23,6 @@ import (
 
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
-	"golang.org/x/crypto/bcrypt"
 	"modernc.org/sqlite"
 	"sneak.berlin/go/neoirc/internal/broker"
 	api "sneak.berlin/go/neoirc/internal/cli/api"
@@ -47,22 +46,25 @@ import (
 const templateDBURL = "file:template?mode=memory&cache=shared"
 
 func TestMain(m *testing.M) {
-	db.SetBcryptCost(bcrypt.MinCost)
+	template := fx.New(
+		fx.NopLogger,
+		fx.Supply(
+			&globals.Globals{},
+			&config.Config{DBURL: templateDBURL},
+		),
+		fx.Provide(logger.New, newTestDB),
+		// fx makes the database only if something asks for it.
+		fx.Invoke(func(*db.Database) {}),
+	)
 
-	template, err := sql.Open("sqlite", templateDBURL)
-	if err != nil {
-		panic(err)
-	}
-
-	err = db.NewTestDatabaseFromConn(template).
-		RunMigrations(context.Background())
+	err := template.Start(context.Background())
 	if err != nil {
 		panic(err)
 	}
 
 	code := m.Run()
 
-	_ = template.Close()
+	_ = template.Stop(context.Background())
 
 	os.Exit(code)
 }
@@ -137,6 +139,7 @@ type testServer struct {
 	t          *testing.T
 	fxApp      *fxtest.App
 	broker     *broker.Broker
+	dbURL      string
 	database   *db.Database
 	handlers   *handlers.Handlers
 }
@@ -231,6 +234,7 @@ func newTestServerWith(
 		t:          t,
 		fxApp:      app,
 		broker:     brk,
+		dbURL:      dbURL,
 		database:   database,
 		handlers:   hdlr,
 	}
@@ -1887,15 +1891,12 @@ func TestFailedLogoutChangesNothing(t *testing.T) {
 
 	_, lastID := tserver.pollMessages(observerToken, 0)
 
-	_, err := tserver.database.GetDB().ExecContext(t.Context(),
+	dbtest.Exec(t, tserver.dbURL,
 		`CREATE TRIGGER keep_sessions
 		 BEFORE DELETE ON sessions
 		 BEGIN
 		   SELECT RAISE(ABORT, 'sessions are kept');
 		 END`)
-	if err != nil {
-		t.Fatal(err)
-	}
 
 	resp, err := doRequestAuth(
 		t, http.MethodPost, tserver.url(apiLogout), token, nil,