Clean up sloghandler testing and restore coverage

This commit retools the way we do testing of sloghandler. It moves the
test implementations of LogSink and SlogSink into new files and then
runs the standard slog tests against them, which excercises both the
SlogSlink and plain LogSink paths.

It also retools sloghandler to handle slog Attrs a bit better and fixes
some of the previous exceptions to slog's standard test.  It still
doesn't hande groups "properly", but this is simpler and users who
really need slog support should use an impl that supports SlogSink (e.g.
funcr).

logr_test.go

@@ -24,85 +24,6 @@ import (
 	"testing"
 )
 
-// testLogSink is a Logger just for testing that calls optional hooks on each method.
-type testLogSink struct {
-	fnInit       func(ri RuntimeInfo)
-	fnEnabled    func(lvl int) bool
-	fnInfo       func(lvl int, msg string, kv ...any)
-	fnError      func(err error, msg string, kv ...any)
-	fnWithValues func(kv ...any)
-	fnWithName   func(name string)
-
-	withValues []any
-
-	// testSlogSink contains some additional fields if (and only if) slog is supported by Go.
-	// See logr_slog_test.go.
-	//nolint:unused // Only unused with Go < 1.21.
-	testSlogSink
-}
-
-var _ LogSink = &testLogSink{}
-
-func (l *testLogSink) Init(ri RuntimeInfo) {
-	if l.fnInit != nil {
-		l.fnInit(ri)
-	}
-}
-
-func (l *testLogSink) Enabled(lvl int) bool {
-	if l.fnEnabled != nil {
-		return l.fnEnabled(lvl)
-	}
-	return false
-}
-
-func (l *testLogSink) Info(lvl int, msg string, kv ...any) {
-	if l.fnInfo != nil {
-		l.fnInfo(lvl, msg, kv...)
-	}
-}
-
-func (l *testLogSink) Error(err error, msg string, kv ...any) {
-	if l.fnError != nil {
-		l.fnError(err, msg, kv...)
-	}
-}
-
-func (l *testLogSink) WithValues(kv ...any) LogSink {
-	if l.fnWithValues != nil {
-		l.fnWithValues(kv...)
-	}
-	out := *l
-	n := len(out.withValues)
-	out.withValues = append(out.withValues[:n:n], kv...)
-	return &out
-}
-
-func (l *testLogSink) WithName(name string) LogSink {
-	if l.fnWithName != nil {
-		l.fnWithName(name)
-	}
-	out := *l
-	return &out
-}
-
-type testCallDepthLogSink struct {
-	testLogSink
-	callDepth       int
-	fnWithCallDepth func(depth int)
-}
-
-var _ CallDepthLogSink = &testCallDepthLogSink{}
-
-func (l *testCallDepthLogSink) WithCallDepth(depth int) LogSink {
-	if l.fnWithCallDepth != nil {
-		l.fnWithCallDepth(depth)
-	}
-	out := *l
-	out.callDepth += depth
-	return &out
-}
-
 func TestNew(t *testing.T) {
 	calledInit := 0