Add first pass at interop tests

tonic-interop/build.rs

@@ -0,0 +1,11 @@
+fn main() {
+    let files = &["proto/grpc/testing/test.proto"];
+    let dirs = &["proto/grpc/testing"];
+
+    tonic_build::compile_protos(files, dirs).unwrap();
+
+    // prevent needing to rebuild if files (or deps) haven't changed
+    for file in files {
+        println!("cargo:rerun-if-changed={}", file);
+    }
+}