ktesting: support capturing log output

This is a feature of the underlying k8s.io/klog/v2/ktesting which is
useful also when using the Kubernetes ktesting.

Author: pohly

test/utils/ktesting/initoption/initoption.go

@@ -28,3 +28,13 @@ func PerTestOutput(enabled bool) InitOption {
 		c.PerTestOutput = enabled
 	}
 }
+
+// BufferLogs controls whether log entries are captured in memory in addition
+// to being printed. Off by default. Unit tests that want to verify that
+// log entries are emitted as expected can turn this on and then retrieve
+// the captured log through the Underlier LogSink interface.
+func BufferLogs(enabled bool) InitOption {
+	return func(c *internal.InitConfig) {
+		c.BufferLogs = enabled
+	}
+}

test/utils/ktesting/internal/config.go

@@ -18,4 +18,5 @@ package internal
 
 type InitConfig struct {
 	PerTestOutput bool
+	BufferLogs    bool
 }

test/utils/ktesting/tcontext.go

@@ -36,7 +36,8 @@ import (
 )
 
 // Underlier is the additional interface implemented by the per-test LogSink
-// behind [TContext.Logger].
+// behind [TContext.Logger]. Together with [initoption.BufferLogs] it can be
+// used to capture log output in memory to check it in tests.
 type Underlier = ktesting.Underlier
 
 // CleanupGracePeriod is the time that a [TContext] gets canceled before the
@@ -245,6 +246,7 @@ func Init(tb TB, opts ...InitOption) TContext {
 			}),
 			ktesting.VerbosityFlagName("v"),
 			ktesting.VModuleFlagName("vmodule"),
+			ktesting.BufferLogs(c.BufferLogs),
 		)
 
 		// Copy klog settings instead of making the ktesting logger