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. Handler tests and the relay tests' aliceWithHTTPClient
helper need a password only to log in, so they store one hashed at
bcrypt.MinCost. Tests of PASS itself keep the real cost.

Model: opus-5-5

internal/dbtest/dbtest.go

@@ -1,5 +1,5 @@
-// Package dbtest gives each test a database of its own. Only tests
-// may import it.
+// Package dbtest gives each test a database of its own and runs raw
+// SQL on it. Only tests may import it.
 package dbtest
 
 import (
@@ -37,3 +37,23 @@ func NewURL(tb testing.TB) string {
 
 	return dbURL
 }
+
+// Exec runs one raw SQL statement on the database at dbURL, over a
+// connection of its own. A test uses it for what the methods of
+// db.Database cannot do, such as creating a trigger that makes a
+// write fail.
+func Exec(tb testing.TB, dbURL, statement string) {
+	tb.Helper()
+
+	pool, err := sql.Open("sqlite", dbURL)
+	if err != nil {
+		tb.Fatalf("open test database: %v", err)
+	}
+
+	defer func() { _ = pool.Close() }()
+
+	_, err = pool.ExecContext(tb.Context(), statement)
+	if err != nil {
+		tb.Fatalf("run %q: %v", statement, err)
+	}
+}