[KeyManager] Fix parallel tests

Ref the code in https://github.com/NilanjanDaw/go-tpm-tools/blob/e114666a20893802fd44ab85b7402d3dc90f4f68/keymanager/workload_service/server_test.go#L1028-L1031
```go
oldTimeout := ClaimsResponseTimeout
ClaimsResponseTimeout = 10 * time.Millisecond
defer func() { ClaimsResponseTimeout = oldTimeout }()
```
Based on comment https://github.com/google/go-tpm-tools/pull/681#discussion_r2888714738 if we ever add t.Parallel() it could result in flaky tests.

In future we can make this configurable field in Server struct

Start with keymanager/workload_service/server_test.go around lines 1028-1031 and review the ClaimsResponseTimeout setup. Check the discussion on pull request 681 for the parallel-test concern, then make the timeout handling safe for parallel tests and verify the affected test behavior.

Written by the indexing model from the issue text.