Debug Windows ECH test flake

[cpu]

The Windows ech test target seems flaky. Roughly half the time it fails with output like:

  research.cloudflare.com.ech.configs.bin
  client[1848]: using the platform verifier for certificate verification.
  client[1848]: using ECH config lists 'research.cloudflare.com.ech.configs.bin'
  client[1848]: no compatible/valid ECH configs found in 'research.cloudflare.com.ech.configs.bin'
  client[1848]: failed to configure ECH with any provided config files

That's pretty odd since we find compatible configs reliably with the Linux and Apple versions of this test. It's not clear if we're putting junk in the .bin file as a result of an issue with ech-fetch, or if there's something wrong on the rustls-ffi/rustls side. More debug output & reproducing it outside of CI are required.

[cpu]

Very odd! I booted into Windows this morning to try and reproduce and: it worked fine. Then I kicked the CI jobs that were failing with this error yesterday and they also passed 🤷 I thought that meant the problem was gone but while preparing #517 it started happening again on that branch.

I'll keep digging.