tests/tester: Do not build as tests

mod tester does not contains any tests, so do not build the module as
containing tests. Instead use the mod.rs approach described in
https://doc.rust-lang.org/book/ch11-03-test-organization.html#submodules-in-integration-tests.

tests/tester/mod.rs

@@ -0,0 +1,102 @@
+use std::env;
+use std::fs::{self, File};
+use std::io::Read;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+use tempfile::TempDir;
+
+use git2::build::CheckoutBuilder;
+use git2::Repository;
+use git2::Signature;
+
+pub struct BatTester {
+    /// Temporary working directory
+    temp_dir: TempDir,
+
+    /// Path to the *bat* executable
+    exe: PathBuf,
+}
+
+impl BatTester {
+    pub fn test_snapshot(&self, name: &str, style: &str) {
+        let output = Command::new(&self.exe)
+            .current_dir(self.temp_dir.path())
+            .args(&[
+                "sample.rs",
+                "--no-config",
+                "--paging=never",
+                "--color=never",
+                "--decorations=always",
+                "--terminal-width=80",
+                &format!("--style={}", style),
+            ])
+            .output()
+            .expect("bat failed");
+
+        // have to do the replace because the filename in the header changes based on the current working directory
+        let actual = String::from_utf8_lossy(&output.stdout)
+            .as_ref()
+            .replace("tests/snapshots/", "");
+
+        let mut expected = String::new();
+        let mut file = File::open(format!("tests/snapshots/output/{}.snapshot.txt", name))
+            .expect("snapshot file missing");
+        file.read_to_string(&mut expected)
+            .expect("could not read snapshot file");
+
+        assert_eq!(expected, actual);
+    }
+}
+
+impl Default for BatTester {
+    fn default() -> Self {
+        let temp_dir = create_sample_directory().expect("sample directory");
+
+        let root = env::current_exe()
+            .expect("tests executable")
+            .parent()
+            .expect("tests executable directory")
+            .parent()
+            .expect("bat executable directory")
+            .to_path_buf();
+
+        let exe_name = if cfg!(windows) { "bat.exe" } else { "bat" };
+        let exe = root.join(exe_name);
+
+        BatTester { temp_dir, exe }
+    }
+}
+
+fn create_sample_directory() -> Result<TempDir, git2::Error> {
+    // Create temp directory and initialize repository
+    let temp_dir = TempDir::new().expect("Temp directory");
+    let repo = Repository::init(&temp_dir)?;
+
+    // Copy over `sample.rs`
+    let sample_path = temp_dir.path().join("sample.rs");
+    println!("{:?}", &sample_path);
+    fs::copy("tests/snapshots/sample.rs", &sample_path).expect("successful copy");
+
+    // Commit
+    let mut index = repo.index()?;
+    index.add_path(Path::new("sample.rs"))?;
+
+    let oid = index.write_tree()?;
+    let signature = Signature::now("bat test runner", "bat@test.runner")?;
+    let tree = repo.find_tree(oid)?;
+    let _ = repo.commit(
+        Some("HEAD"), //  point HEAD to our new commit
+        &signature,   // author
+        &signature,   // committer
+        "initial commit",
+        &tree,
+        &[],
+    );
+    let mut opts = CheckoutBuilder::new();
+    repo.checkout_head(Some(opts.force()))?;
+
+    fs::copy("tests/snapshots/sample.modified.rs", &sample_path).expect("successful copy");
+
+    Ok(temp_dir)
+}