Added tests for the command line application.

net_income_germany_cmd/Cargo.toml

@@ -6,3 +6,7 @@ edition = "2021"
 [dependencies]
 clap = { version = "4.5.4", features = ["deprecated", "derive"] }
 net-income-germany = { path = "../net_income_germany" }
+
+[dev-dependencies]
+assert_cmd = "2.0.14"
+predicates = "3.1.0"

net_income_germany_cmd/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 calculate_for_current_year() -> Result<(), Box<dyn std::error::Error>> {
+    let mut cmd = Command::cargo_bin("net-income-germany-cmd")?;
+
+    cmd.arg("--income").arg("80000");
+    cmd.assert()
+        .success()
+        .stdout(predicate::str::contains(" 48173,"));
+
+    Ok(())
+}