Fix cfg for test

Fix up cfg attributes to work on an xor basis.

Previously, the cfg(any()) attributes would cause issues when
both native-tls and tls features were enabled. Now, https functions
and enum variants will only be created when tls xor native-tls are
enabled. Additionally, a compile error has been added for when
both tls and native-tls features are enabled.

tests/https-agent.rs

@@ -1,7 +1,8 @@
 #[cfg(all(test, any(feature = "tls", feature = "native-tls")))]
 use std::io::Read;
 
-#[cfg(all(test, any(feature = "tls", feature = "native-tls")))]
+#[cfg(any(feature = "tls", feature = "native-tls"))]
+#[test]
 fn tls_connection_close() {
     let agent = ureq::Agent::default().build();
     let resp = agent