Share ReadFile in tests (#4967)

Small cleanup for code sharing. Note, `*ReadFile` will trigger a
CHECK-failure on error; the relevant implementations would previously
have failed silently (empty string).

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

testing/base/file_helpers.cpp

@@ -0,0 +1,25 @@
+// 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/file_helpers.h"
+
+#include <fstream>
+#include <sstream>
+
+namespace Carbon::Testing {
+
+auto ReadFile(std::filesystem::path path) -> ErrorOr<std::string> {
+  std::ifstream file_stream(path);
+  if (file_stream.fail()) {
+    return Error(llvm::formatv("Error opening file: {0}", path));
+  }
+  std::stringstream buffer;
+  buffer << file_stream.rdbuf();
+  if (file_stream.fail()) {
+    return Error(llvm::formatv("Error reading file: {0}", path));
+  }
+  return buffer.str();
+}
+
+}  // namespace Carbon::Testing