Fix TLS E2E test flakiness

[gjcairo]

Some of the E2E tests with TLS enabled were flaky on Darwin.

This PR:

• Slightly modifies the way they are implemented to make them not-flaky
• Modifies the options passed to SecPKCS12Import to make sure the creation of the required SecIdentity happens in memory only and doesn't use the keychain
• Tolerates a broken pipe error when the server fails client cert validation during an mTLS handshake, as it's possible for Network.framework to expose this error instead of the actual SSL validation error on the client

[glbrntt]

Thanks a lot @gjcairo!

[glbrntt]

@gjcairo FYI formatter is unhappy