testing/slogtest: add Run to run cases as subtests

This is an implementation of proposal #61758.

It adds a function to slogtest that runs each test case in a subtest,
instead of running them all at once.

That allows the caller to control which cases are run.

Fixes #61706.
Fixes #61758.

Change-Id: I95108b7b753675203ca7f0f00ccbc242bd9c2a9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/516076
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>

api/next/61758.txt

@@ -0,0 +1 @@
+pkg testing/slogtest, func Run(*testing.T, func(*testing.T) slog.Handler, func(*testing.T) map[string]interface{}) #61758

src/go/build/deps_test.go

@@ -572,15 +572,15 @@ var depsRules = `
 	< testing/iotest
 	< testing/fstest;
 
-	log/slog
-	< testing/slogtest;
-
 	FMT, flag, math/rand
 	< testing/quick;
 
 	FMT, DEBUG, flag, runtime/trace, internal/sysinfo, math/rand
 	< testing;
 
+	log/slog, testing
+	< testing/slogtest;
+
 	FMT, crypto/sha256, encoding/json, go/ast, go/parser, go/token,
 	internal/godebug, math/rand, encoding/hex, crypto/sha256
 	< internal/fuzz;

src/testing/slogtest/run_test.go

@@ -0,0 +1,31 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package slogtest_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"log/slog"
+	"testing"
+	"testing/slogtest"
+)
+
+func TestRun(t *testing.T) {
+	var buf bytes.Buffer
+
+	newHandler := func(*testing.T) slog.Handler {
+		buf.Reset()
+		return slog.NewJSONHandler(&buf, nil)
+	}
+	result := func(t *testing.T) map[string]any {
+		m := map[string]any{}
+		if err := json.Unmarshal(buf.Bytes(), &m); err != nil {
+			t.Fatal(err)
+		}
+		return m
+	}
+
+	slogtest.Run(t, newHandler, result)
+}