Refactor stdout and stderr capturing to a library (#4970)

This should make it easy to add more tests which need the specific
behavior here. It also isolates where we reach into GoogleTest's
internals to a single common place.

testing/base/capture_std_streams.cpp

@@ -0,0 +1,34 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "testing/base/capture_std_streams.h"
+
+#include <gtest/gtest.h>
+
+#include <fstream>
+#include <sstream>
+
+#include "common/ostream.h"
+
+namespace Carbon::Testing::Internal {
+
+// While these are marked as "internal" APIs, they seem to work and be pretty
+// widely used for their exact documented behavior.
+using ::testing::internal::CaptureStderr;
+using ::testing::internal::CaptureStdout;
+using ::testing::internal::GetCapturedStderr;
+using ::testing::internal::GetCapturedStdout;
+
+auto BeginStdStreamCapture() -> void {
+  CaptureStderr();
+  CaptureStdout();
+}
+auto EndStdStreamCapture(std::string& out, std::string& err) -> void {
+  // No need to flush stderr.
+  err = GetCapturedStderr();
+  llvm::outs().flush();
+  out = GetCapturedStdout();
+}
+
+}  // namespace Carbon::Testing::Internal