tests: support multiple HTTPS RRs for ECH configs

[cpu]

Similar to a change in the upstream Rustls ech-client.rs demo we want to be able to process multiple HTTPS records for a given domain, and look at each ECH config list from each record for a potential compatible config. Follow-up to #485.

Mechanically this means:

1. Updating the test/ech_fetch.rs helper to support writing multiple .bin files when there are multiple HTTPS records w/ ECH configs. The tool now outputs to stdout a comma separated list of the files it writes to make it easier to use with the client.c example.

2. Updating the tests/client.c example to treat the ECH_CONFIG_LIST env var as a comma separated list of ECH config lists. We now loop through each and only fail if all of the provided files are unable to be used to configure the client config with a compatible ECH config.

Doing string manipulation with C remains "a delight". For Windows compat we achieve tokenizing the string by the comma delim with a define to call either strtok_r with GCC/clang, or strtok_s with MSCV.

You can test this update with:

ECH_CONFIG_LISTS=$(cargo test --test ech_fetch -- curves1-ng.test.defo.ie /tmp/curves1-ng.test.defo.ie)
RUSTLS_PLATFORM_VERIFIER=1 ECH_CONFIG_LIST="$ECH_CONFIG_LISTS" ./cmake-build-debug/tests/client curves1-ng.test.defo.ie 443 /echstat.php?format=json

If you're unlucky and the first HTTPS record served is the one with invalid configs you should see output like the following showing the client skipping over the .1 config list and using the .2 one instead:

client[188911]: no compatible/valid ECH configs found in '/tmp/curves1-ng.test.defo.ie.1'
client[188911]: using ECH with config list from '/tmp/curves1-ng.test.defo.ie.2'

This logic will also need to be ported into #497. I'll do that shortly but expect we can land this PR first against the pre-existing client.c and then catch 497 up afterwards.

Resolves #503

[cpu]

This feels incremental enough to merge w/o waiting for further review.