chore: fix concurrent CommitQuota transactions for unrelated users/orgs (#15261)

The failure condition being fixed is `w1` and `w2` could belong
to different users, organizations, and templates and still cause a
serializable failure if run concurrently. This is because the old query 
did a `seq scan` on the `workspace_builds` table. Since that is the 
table being updated, we really want to prevent that.

So before this would fail for any 2 workspaces. Now it only fails if
`w1` and `w2` are owned by the same user and organization.

coderd/database/dbtestutil/db.go

@@ -135,7 +135,8 @@ func NewDB(t testing.TB, opts ...Option) (database.Store, pubsub.Pubsub) {
 		if o.dumpOnFailure {
 			t.Cleanup(func() { DumpOnFailure(t, connectionURL) })
 		}
-		db = database.New(sqlDB)
+		// Unit tests should not retry serial transaction failures.
+		db = database.New(sqlDB, database.WithSerialRetryCount(1))
 
 		ps, err = pubsub.New(context.Background(), o.logger, sqlDB, connectionURL)
 		require.NoError(t, err)

coderd/database/dbtestutil/tx.go

@@ -0,0 +1,73 @@
+package dbtestutil
+
+import (
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/coderd/database"
+)
+
+type DBTx struct {
+	database.Store
+	mu       sync.Mutex
+	done     chan error
+	finalErr chan error
+}
+
+// StartTx starts a transaction and returns a DBTx object. This allows running
+// 2 transactions concurrently in a test more easily.
+// Example:
+//
+//	a := StartTx(t, db, opts)
+//	b := StartTx(t, db, opts)
+//
+//	a.GetUsers(...)
+//	b.GetUsers(...)
+//
+//	require.NoError(t, a.Done()
+func StartTx(t *testing.T, db database.Store, opts *database.TxOptions) *DBTx {
+	done := make(chan error)
+	finalErr := make(chan error)
+	txC := make(chan database.Store)
+
+	go func() {
+		t.Helper()
+		once := sync.Once{}
+		count := 0
+
+		err := db.InTx(func(store database.Store) error {
+			// InTx can be retried
+			once.Do(func() {
+				txC <- store
+			})
+			count++
+			if count > 1 {
+				// If you recursively call InTx, then don't use this.
+				t.Logf("InTx called more than once: %d", count)
+				assert.NoError(t, xerrors.New("InTx called more than once, this is not allowed with the StartTx helper"))
+			}
+
+			<-done
+			// Just return nil. The caller should be checking their own errors.
+			return nil
+		}, opts)
+		finalErr <- err
+	}()
+
+	txStore := <-txC
+	close(txC)
+
+	return &DBTx{Store: txStore, done: done, finalErr: finalErr}
+}
+
+// Done can only be called once. If you call it twice, it will panic.
+func (tx *DBTx) Done() error {
+	tx.mu.Lock()
+	defer tx.mu.Unlock()
+
+	close(tx.done)
+	return <-tx.finalErr
+}