chore(test): Use library crate in test (#2201)

Author: tottoto

tests/ambiguous_methods/src/main.rs renamed to tests/ambiguous_methods/src/lib.rs

@@ -2,7 +2,3 @@
 extern crate tonic;
 
 tonic::include_proto!("ambiguous_methods");
-
-fn main() {
-    println!("Hello, world!");
-}

tests/root-crate-path/src/main.rs renamed to tests/root-crate-path/src/lib.rs

@@ -13,7 +13,3 @@ pub mod foo {
         }
     }
 }
-
-fn main() {
-    println!("Hello, world!");
-}

tests/stream_conflict/src/main.rs renamed to tests/stream_conflict/src/lib.rs

@@ -1,7 +1,3 @@
 mod stream_conflict {
     tonic::include_proto!("stream_conflict");
 }
-
-fn main() {
-    println!("Hello, world!");
-}