Preallocate auth.NewAnonymousCredentials() in contexttest.TestContext.
Preallocate auth.NewAnonymousCredentials() in contexttest.TestContext.

Otherwise every call to, say, fs.ContextCanAccessFile() in a benchmark
using contexttest allocates new auth.Credentials, a new
auth.UserNamespace, ...

PiperOrigin-RevId: 254261051

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor