Put a time limit even on tests expected to hang

Without this, in some cases when cargo-mutants is itself mutated or
buggy, test subprocesses can hang around forever, which is not useful.

With this change, they will eventually exit, but after long enough that
timeout handling can still be tested.

testdata/tree/already_hangs/src/lib.rs

@@ -6,7 +6,10 @@ use std::thread::sleep;
 use std::time::Duration;
 
 pub fn infinite_loop() {
-    for i in 0.. {
+    // Not really infinite, so that orphaned processes don't hang around forever.
+    // They shouldn't normally happen, but they might when cargo-mutants is itself
+    // being mutation tested, or has a bug, etc.
+    for i in 0..600 {
         println!("{}", i);
         sleep(Duration::from_secs(1));
     }

testdata/tree/hang_avoided_by_attr/src/lib.rs

@@ -1,5 +1,7 @@
 //! An example of a function we should not mutate because it will hang.
 
+use std::time::{Duration, Instant};
+
 /// If mutated to return false, the program will spin forever.
 ///
 /// Ideally and eventually, cargo-mutants should stop it after a timeout,
@@ -10,11 +12,15 @@ fn should_stop() -> bool {
 }
 
 pub fn controlled_loop() {
+    let start = Instant::now();
     for i in 0.. {
         println!("{}", i);
         if should_stop() {
             break;
         }
+        if start.elapsed() > Duration::from_secs(60 * 5) {
+            panic!("timed out");
+        }
     }
 }
 

testdata/tree/hang_when_mutated/src/lib.rs

@@ -4,6 +4,7 @@
 //! lets us test the case where that has not yet been fixed.
 
 use std::sync::atomic::{AtomicBool, Ordering};
+use std::time::{Duration, Instant};
 
 static TRIGGER: AtomicBool = AtomicBool::new(false);
 
@@ -18,13 +19,20 @@ fn should_stop() -> bool {
 
 /// Runs until `should_stop` returns true, and then returns the number
 /// of iterations.
+///
+/// Also stops after a few minutes anyhow, so that if the timeouts are not
+/// properly implemented, the child process doesn't hang around forever.
 pub fn controlled_loop() -> usize {
+    let start = Instant::now();
     for i in 1.. {
         println!("{}", i);
         if should_stop() {
             return i;
         }
         std::thread::sleep(std::time::Duration::from_millis(100));
+        if start.elapsed() > Duration::from_secs(60 * 5) {
+            panic!("timed out");
+        }
     }
     unreachable!();
 }

tests/snapshots/cli__check_tree_with_mutants_skip.snap

@@ -5,6 +5,6 @@ expression: stdout
 Found 1 mutant to test
 Copy source to scratch directory ... done
 Unmutated baseline ... ok
-src/lib.rs:12: replace controlled_loop with () ... ok
+src/lib.rs:14: replace controlled_loop with () ... ok
 1 mutant tested: 1 succeeded
 

tests/snapshots/cli__list_mutants_in_all_trees_as_json.snap

@@ -125,7 +125,7 @@ expression: buf
   {
     "package": "cargo-mutants-testdata-hang-avoided-by-attr",
     "file": "src/lib.rs",
-    "line": 12,
+    "line": 14,
     "function": "controlled_loop",
     "return_type": "",
     "replacement": "()"
@@ -140,23 +140,23 @@ expression: buf
   {
     "package": "cargo-mutants-testdata-hang-when-mutated",
     "file": "src/lib.rs",
-    "line": 11,
+    "line": 12,
     "function": "should_stop",
     "return_type": "-> bool",
     "replacement": "true"
   },
   {
     "package": "cargo-mutants-testdata-hang-when-mutated",
     "file": "src/lib.rs",
-    "line": 11,
+    "line": 12,
     "function": "should_stop",
     "return_type": "-> bool",
     "replacement": "false"
   },
   {
     "package": "cargo-mutants-testdata-hang-when-mutated",
     "file": "src/lib.rs",
-    "line": 21,
+    "line": 25,
     "function": "controlled_loop",
     "return_type": "-> usize",
     "replacement": "Default::default()"

tests/snapshots/cli__list_mutants_in_all_trees_as_text.snap

@@ -57,15 +57,15 @@ src/bin/factorial.rs:7: replace factorial -> u32 with Default::default()
 ## testdata/tree/hang_avoided_by_attr
 
 ```
-src/lib.rs:12: replace controlled_loop with ()
+src/lib.rs:14: replace controlled_loop with ()
 ```
 
 ## testdata/tree/hang_when_mutated
 
 ```
-src/lib.rs:11: replace should_stop -> bool with true
-src/lib.rs:11: replace should_stop -> bool with false
-src/lib.rs:21: replace controlled_loop -> usize with Default::default()
+src/lib.rs:12: replace should_stop -> bool with true
+src/lib.rs:12: replace should_stop -> bool with false
+src/lib.rs:25: replace controlled_loop -> usize with Default::default()
 ```
 
 ## testdata/tree/insta