test: enable webpki TLS feature for test

[arjan-bal]

Fixes: #2801

This change enables the tls-webpki-roots feature for integration tests that verify the identity of github.com.

This issue wasn't caught by CI because our CI configuration currently builds with all feature flags enabled. Please see the attached issue for details on the root cause.

Tested

Verified that cargo test on the workspace root works.

[dfawley]

Should we change the CI to not pass --all-features when running tests to ensure that a bare cargo test will continue to pass?

@LucioFranco?

[arjan-bal]

If we don't use --all-features, unit tests for non-default features would get skipped. For example, the compression tests that require optional codecs would not run.

[dfawley]

Is it too much of a waste to run both with and without --all-features? I'm thinking the only way we can guarantee that potential contributors will be able to come to our repo and run cargo test and see a pass is if we run it that way in CI.