Update invalid cert testing instructions (#536)
Update invalid cert testing instructions (#536)

Chrome feature flag AllowSignedHTTPExchangeCertsWithoutExtension was
removed in M89. https://bugs.chromium.org/p/chromium/issues/detail?id=862003

Fortunately, the ability to test SXGs using certificates lacking the
CanSignHttpExchanges(Draft) extension is still possible thanks to an
update to the --ignore-certificate-errors-spki-list command line flag.
https://bugs.chromium.org/p/chromium/issues/detail?id=956471

Update instructions to include an example of 1) how to launch Chrome
using this flag, and 2) how to generate a certificate public hash
suitable for use by this flag.

Additional information about flag:
https://chromium.googlesource.com/chromium/src/+/refs/tags/89.0.4389.130/services/network/public/cpp/network_switches.cc#24
ampproject/amppackager,ampproject/amppackager,ampproject/amppackager