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 that need a password only to log in store
one hashed at bcrypt.MinCost. Tests of PASS itself keep the real cost.

Model: opus-5-5

internal/ircserver/relay_test.go

@@ -8,8 +8,6 @@ import (
 	"testing"
 	"time"
 
-	"golang.org/x/crypto/bcrypt"
-	"sneak.berlin/go/neoirc/internal/dbtest"
 	"sneak.berlin/go/neoirc/internal/ircserver"
 	"sneak.berlin/go/neoirc/internal/service"
 )
@@ -585,10 +583,8 @@ func ownSessionCopies(
 
 // aliceWithHTTPClient connects alice to the IRC listener and
 // has her join #chan, which bob is on too, then adds an HTTP API
-// client to her session with a password, as POST /api/v1/login
+// client to her session with the password her connection set,
-// does. It stores that password without sending PASS. It hashes
+// as POST /api/v1/login does. It returns her connection, bob's
-// at bcrypt.MinCost, and a login compares at the cost stored in
-// the hash, so both are fast. It returns her connection, bob's
 // connection, her session and her HTTP API client.
 func aliceWithHTTPClient(
 	t *testing.T, env *testEnv,
@@ -598,24 +594,15 @@ func aliceWithHTTPClient(
 	const password = "alicepass"
 
 	alice := env.dial(t)
+	alice.send("PASS " + password)
 	alice.register("alice")
+	// Registration stores the password before the JOIN runs.
 	alice.joinAndDrain("#chan")
 
 	bob := env.dial(t)
 	bob.register("bob")
 	bob.joinAndDrain("#chan")
 
-	hash, err := bcrypt.GenerateFromPassword(
-		[]byte(password), bcrypt.MinCost,
-	)
-	if err != nil {
-		t.Fatalf("hash password: %v", err)
-	}
-
-	dbtest.Exec(t, env.dbURL,
-		"UPDATE sessions SET password_hash = '"+string(hash)+
-			"' WHERE nick = 'alice'")
-
 	sessionID, httpClientID, _, err := env.database.LoginUser(
 		t.Context(), "alice", password, "127.0.0.1", "localhost",
 	)