Add some integration test coverage

tests/README.md

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+Tests
+---
+
+This directory contains test cases for this code. Test cases:
+* `passing-workspace` is a minimal passing workspace (one crate).
+* `failing-workspace` is a minimal failing workspace (one crate).
+
+`test.rs` runs each workspace as a test case and ensures the binary passes or fails as desired.

tests/failing-workspace/Cargo.toml

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+[workspace]
+members = ["test-crate"]
+
+[workspace.lints.rustc]
+missing-docs = "deny"

tests/failing-workspace/test-crate/Cargo.toml

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+[package]
+name = "test-crate"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

tests/passing-workspace/Cargo.toml

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+[workspace]
+members = ["test-crate"]
+
+[workspace.lints.rustc]
+missing-docs = "deny"

tests/passing-workspace/test-crate/Cargo.toml

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+[package]
+name = "test-crate"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+
+[lints]
+workspace = true

tests/test.rs

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+use std::process::Command;
+
+#[test]
+fn test_passing_workspace() {
+    let output = Command::new(env!("CARGO_BIN_EXE_cargo-workspace-lints"))
+        .arg("workspace-lints")
+        .arg("tests/passing-workspace/Cargo.toml")
+        .output()
+        .expect("Failed to run program");
+    assert!(output.status.success(), "{output:?}");
+}
+
+#[test]
+fn test_failing_workspace() {
+    let output = Command::new(env!("CARGO_BIN_EXE_cargo-workspace-lints"))
+        .arg("workspace-lints")
+        .arg("tests/failing-workspace/Cargo.toml")
+        .output()
+        .expect("Failed to run program");
+    assert!(!output.status.success(), "{output:?}");
+    // Check that it mentions the crate that fails
+    assert!(String::from_utf8_lossy(&output.stderr).contains("tests/failing-workspace/test-crate"));
+}