Fix QuicheBufferAllocatorTest

Apparently, the regex parser Chromium gtest uses is insufficiently advanced to parse "[+]".

PiperOrigin-RevId: 446513567

quiche/common/quiche_buffer_allocator_test.cc

diff --git a/quiche/common/quiche_buffer_allocator_test.cc b/quiche/common/quiche_buffer_allocator_test.cc
index e641481..974d6fc 100644
--- a/quiche/common/quiche_buffer_allocator_test.cc
+++ b/quiche/common/quiche_buffer_allocator_test.cc
@@ -133,7 +133,7 @@
       QuicheBuffer::CopyFromIovec(&allocator, &iov[0],
                                   /* iov_count = */ 2, /* iov_offset = */ 2,
                                   /* buffer_length = */ 10),
-      "iov_offset [+] buffer_length larger than iovec total size");
+      R"(iov_offset \+ buffer_length larger than iovec total size)");
 }
 
 }  // anonymous namespace