test: Don't fail if troff is missing

tests/testsuite/demo.rs

@@ -5,6 +5,10 @@ use pretty_assertions::assert_eq;
 fn demo() {
     use roff::*;
 
+    if !has_command("troff") {
+        return;
+    }
+
     let page = Roff::new()
         .control("TH", ["CORRUPT", "1"])
         .control("SH", ["NAME"])
@@ -63,3 +67,52 @@ fn roff_to_ascii(input: &str) -> String {
         .read()
         .unwrap()
 }
+
+pub(crate) fn has_command(command: &str) -> bool {
+    let output = match std::process::Command::new(command)
+        .arg("--version")
+        .output()
+    {
+        Ok(output) => output,
+        Err(e) => {
+            // CI is expected to support all of the commands
+            if is_ci() && cfg!(target_os = "linux") {
+                panic!(
+                    "expected command `{}` to be somewhere in PATH: {}",
+                    command, e
+                );
+            }
+            return false;
+        }
+    };
+    if !output.status.success() {
+        panic!(
+            "expected command `{}` to be runnable, got error {}:\n\
+            stderr:{}\n\
+            stdout:{}\n",
+            command,
+            output.status,
+            String::from_utf8_lossy(&output.stderr),
+            String::from_utf8_lossy(&output.stdout)
+        );
+    }
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    println!(
+        "$ {command} --version
+{}",
+        stdout
+    );
+    if cfg!(target_os = "macos") && stdout.starts_with("GNU bash, version 3") {
+        return false;
+    }
+
+    true
+}
+
+/// Whether or not this running in a Continuous Integration environment.
+fn is_ci() -> bool {
+    // Consider using `tracked_env` instead of option_env! when it is stabilized.
+    // `tracked_env` will handle changes, but not require rebuilding the macro
+    // itself like option_env does.
+    option_env!("CI").is_some() || option_env!("TF_BUILD").is_some()
+}