Add flush file test

tests/flush.rs

@@ -0,0 +1,31 @@
+use std::fs;
+use std::time::{SystemTime, UNIX_EPOCH};
+
+use fastlog::LogBuilder;
+
+#[test]
+fn flush_writes_message_to_file() {
+    let path = std::env::temp_dir().join(format!(
+        "fastlog-test-{}.log",
+        SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .as_nanos()
+    ));
+
+    let mut builder = LogBuilder::new();
+    builder.file(path.clone());
+    builder
+        .build()
+        .expect("failed to build logger")
+        .init()
+        .expect("failed to init logger");
+
+    log::info!("flushed message");
+    log::logger().flush();
+
+    let content = fs::read_to_string(&path).expect("failed to read log file");
+    assert!(content.contains("flushed message"));
+
+    fs::remove_file(path).expect("failed to remove log file");
+}