[`git cherry-pick 4a39e2b67d4cddf58b0ea16dd821a04ee2240058`, with support
for Edition 2018 added by Brian.]
This commit introduces the Netflix BetterTLS[0]'s path building test
suite to the webpki integration tests.
This project has a test runner for Rustls that will stand up TLS servers
to exercise these tests but:
* It requires Go.
* It needs Rustls in order to do a full TLS handshake with the test
servers.
* It's slower than testing the path building directly without the TLS
bits.
To avoid these issues this commit takes a different approach and vendors
the exported path building test suite. This is a supported feature[1] of
the upstream project and allow us to directly test webpki's path
building against the test suite without needing Rustls or Go.
[0]: https://github.com/Netflix/bettertls
[1]: https://github.com/Netflix/bettertls#exporting-tests-to-run-outside-of-the-bettertls-executor