Make tests green on systems with different number of physical CPUs

Also pretty print the assert_eq call of the big output.txt comparisson to
be actually able to see where things fail with diff.

Cargo.lock

@@ -21,6 +21,7 @@ dependencies = [
  "clap",
  "futures",
  "num_cpus",
+ "pretty_assertions",
  "rand",
  "toml",
  "walkdir",

src/alejandra_cli/Cargo.toml

@@ -30,3 +30,6 @@ license = "Unlicense"
 name = "alejandra_cli"
 repository = "https://github.com/kamadorueda/alejandra"
 version = "3.1.0"
+
+[dev-dependencies]
+pretty_assertions = "1.3.0"

src/alejandra_cli/src/cli.rs

@@ -1,3 +1,4 @@
+use std::env;
 use std::fs::read_to_string;
 use std::io::Read;
 
@@ -145,8 +146,15 @@ pub fn main() -> ! {
     let include: Vec<&str> =
         args.include.iter().map(String::as_str).collect::<Vec<&str>>();
 
-    let threads =
-        args.threads.map_or_else(num_cpus::get_physical, Into::<usize>::into);
+    // Try CLI value, then env var, then fall back to number of physical CPUs.
+    let threads: usize = if let Some(cli_threads) = args.threads {
+        cli_threads.into() // convert u8 to usize
+    } else {
+        env::var("ALEJANDRA_THREADS")
+            .ok()
+            .and_then(|v| v.parse::<usize>().ok())
+            .unwrap_or_else(num_cpus::get_physical)
+    };
 
     let verbosity = match args.quiet {
         0 => Verbosity::Everything,

src/alejandra_cli/tests/cli.rs

@@ -4,6 +4,8 @@ use std::path::PathBuf;
 use std::process::Command;
 use std::process::Stdio;
 
+use pretty_assertions::assert_eq;
+
 #[derive(Debug)]
 struct TestCase {
     args:  &'static [&'static str],
@@ -126,6 +128,7 @@ fn cases() {
         output_got.push_str(&format!("args: {:?}\n", case.args));
 
         let mut child = Command::new("cargo")
+            .env("ALEJANDRA_THREADS", "4")
             .args(["run", "--quiet", "--"])
             .args(case.args)
             .stdin(Stdio::piped())