Feat: Add integration test for file not exist.

tests/cli.rs

@@ -0,0 +1,15 @@
+use assert_cmd::prelude::*; // Add methods on commands
+use predicates::prelude::*; // Used for writing assertions
+use std::process::Command; // Run programs
+
+#[test]
+fn file_doesnt_exist() -> Result<(), Box<dyn std::error::Error>> {
+    let mut cmd = Command::cargo_bin("grrs")?;
+
+    cmd.arg("foobar").arg("test/file/doesnt/exist");
+    cmd.assert()
+        .failure()
+        .stderr(predicate::str::contains("could not read file"));
+
+    Ok(())
+}