Stop the absl::Cord destructor from running on the constinit cord in
CordTest.ConstinitConstructor. This allows inspecting the cord at any point
while the test is exiting, which is important for the semantics of the test.

PiperOrigin-RevId: 448327386
Change-Id: Icef9faa2b63f1f0ae60b3430dcf6184f5dead885