Wait for rbuilder running before running integration test (#160)

## 📝 Summary

The new integration test introduced in #143 requires the rbuilder to be
running to work. However, the playground setup was not waiting for it.
This PR fixes that. It adds a loop after the rbuilder creation that
checks in the output log for a specific log "RPC server job: started"
that signals that the builder is running and the test can start.

## 💡 Motivation and Context

<!--- (Optional) Why is this change required? What problem does it
solve? Remove this section if not applicable. -->

---

## ✅ I have completed the following steps:

* [x] Run `make lint`
* [x] Run `make test`
* [x] Added tests (if applicable)

crates/rbuilder/src/integration/playground.rs

@@ -15,7 +15,8 @@ use std::{
     path::PathBuf,
     process::{Child, Command},
     str::FromStr,
-    time::SystemTime,
+    thread,
+    time::{Instant, SystemTime},
 };
 use time::{format_description, OffsetDateTime};
 use url::Url;
@@ -26,6 +27,7 @@ pub enum PlaygroundError {
     BinaryNotFound,
     SetupError,
     IntegrationPathNotFound,
+    Timeout,
 }
 
 pub struct Playground {
@@ -77,7 +79,7 @@ impl Playground {
         let mut log_path = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
         log_path.push(format!("../../integration_logs/{}.log", name));
 
-        let log = open_log_file(log_path).map_err(|_| PlaygroundError::SetupError)?;
+        let log = open_log_file(log_path.clone()).map_err(|_| PlaygroundError::SetupError)?;
         let stdout = log.try_clone().map_err(|_| PlaygroundError::SetupError)?;
         let stderr = log.try_clone().map_err(|_| PlaygroundError::SetupError)?;
 
@@ -94,6 +96,26 @@ impl Playground {
             },
         }?;
 
+        let start = Instant::now();
+        loop {
+            if start.elapsed().as_secs() > 10 {
+                return Err(PlaygroundError::Timeout);
+            }
+
+            // from the log file, check if the server has started
+            // by checking if the string "RPC server job: started" is present
+            let mut file = File::open(&log_path).map_err(|_| PlaygroundError::SetupError)?;
+            let mut contents = String::new();
+            file.read_to_string(&mut contents)
+                .map_err(|_| PlaygroundError::SetupError)?;
+
+            if contents.contains("RPC server job: started") {
+                break;
+            }
+
+            thread::sleep(std::time::Duration::from_millis(100));
+        }
+
         Ok(Self { builder })
     }