logtest: add Transform (#6794)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6491
Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6489

Towards https://github.com/open-telemetry/opentelemetry-go/issues/6341

Per
https://github.com/open-telemetry/opentelemetry-go/pull/6774#discussion_r2091424563

log/logtest/example_test.go

@@ -0,0 +1,55 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package logtest_test
+
+import (
+	"context"
+	"testing"
+	"time"
+
+	"go.opentelemetry.io/otel/log"
+	"go.opentelemetry.io/otel/log/logtest"
+)
+
+func Example() {
+	t := &testing.T{} // Provided by testing framework.
+
+	// Create a recorder.
+	rec := logtest.NewRecorder()
+
+	// Emit a log record (code under test).
+	l := rec.Logger("Example")
+	ctx := context.Background()
+	r := log.Record{}
+	r.SetTimestamp(time.Now())
+	r.SetSeverity(log.SeverityInfo)
+	r.SetBody(log.StringValue("Hello there"))
+	r.AddAttributes(log.String("foo", "bar"))
+	r.AddAttributes(log.Int("n", 1))
+	l.Emit(ctx, r)
+
+	// Verify that the expected and actual log records match.
+	want := logtest.Recording{
+		logtest.Scope{Name: "Example"}: []logtest.Record{
+			{
+				Context:  context.Background(),
+				Severity: log.SeverityInfo,
+				Body:     log.StringValue("Hello there"),
+				Attributes: []log.KeyValue{
+					log.Int("n", 1),
+					log.String("foo", "bar"),
+				},
+			},
+		},
+	}
+	got := rec.Result()
+	logtest.AssertEqual(t, want, got,
+		// Ignore Timestamps.
+		logtest.Transform(func(time.Time) time.Time {
+			return time.Time{}
+		}),
+	)
+	// Output:
+	//
+}