Move unit tests inside conditionally compiled mod tests { } blocks

Idiomatic rust organizes unit tests into `mod tests { }` blocks
using conditional compilation `[cfg(test)]` to decide whether to
compile the code in that block.

This commit moves "bare" test functions into such blocks, and puts
the block at the bottom of respective file.

src/proxy.rs

@@ -171,8 +171,7 @@ Proxy-Connection: Keep-Alive\r\n\
 
 #[cfg(test)]
 mod tests {
-    use super::Proto;
-    use super::Proxy;
+    use super::*;
 
     #[test]
     fn parse_proxy_fakeproto() {