Merge #18691: test: add wait_for_cookie_credentials() to framework for rpcwait tests

92fe537cf704dfb4ae830c8c8b382f08c4893e65 test: fix intermittent race condition in interface_bitcoin_cli.py (Jon Atack)
c648e636b2f230db5f1d1137088685f74ae42454 test: add wait_for_cookie_credentials() to test framework (Jon Atack)

Pull request description:

  This PR adds a `wait_for_cookie_credentials()` method to the test framework and calls it before the `-rpcwait` tests, to avoid an intermittent race condition on the CI run with Valgrind where the cookie file isn't written yet when the CLI call with `-rpcwait` arrives to `get_auth_cookie()`.

  To reproduce/test, build with
  ```diff
  diff --git a/src/httprpc.cpp b/src/httprpc.cpp
  index 60c4d06f12..3dd06c4758 100644
  --- a/src/httprpc.cpp
  +++ b/src/httprpc.cpp
  @@ -291,6 +291,7 @@ static bool InitRPCAuthentication()
   bool StartHTTPRPC()
   {
       LogPrint(BCLog::RPC, "Starting HTTP RPC server\n");
  +    UninterruptibleSleep(std::chrono::seconds{11});
       if (!InitRPCAuthentication())
  ```
  then run the test normally and with valgrind
  ```
  test/functional/interface_bitcoin_cli.py -l debug
  valgrind test/functional/interface_bitcoin_cli.py -l debug
  ```
  Thanks to Marco Falke for all the help. Closes #18684.

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Tree-SHA512: 1b76635b5b1d6b05138affef7ab788aa3bc3fc75b0c69ba778ecdf81063cfe02a8dd7667cfd63a6c6e19b2dac47d7a8b755e334d8af5c0ab9d4026808ee96c83

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