Skip node2.is_running() on Windows

`node2.is_running()` can return `true` on Windows, even though `node2`
has logged a panic. This cleanup code only runs if `node2` fails to panic
and exit as expected. So it's ok for us to skip it.

See #1781 for details.

zebra-test/src/command.rs

@@ -355,6 +355,11 @@ impl<T> TestChild<T> {
     }
 
     /// Is this process currently running?
+    ///
+    /// ## BUGS
+    ///
+    /// On Windows, this function can return `true` for processes that have
+    /// panicked. See #1781.
     pub fn is_running(&mut self) -> bool {
         matches!(self.child.try_wait(), Ok(None))
     }

zebrad/tests/acceptance.rs

@@ -1231,12 +1231,18 @@ where
     let output1 = node1.wait_with_output();
     let (output1, mut node2) = node2.kill_on_error(output1)?;
 
-    // node2 should have panicked due to a conflict. Kill it here
-    // anyway, so it doesn't outlive the test on error.
+    // node2 should have panicked due to a conflict. Kill it here anyway, so it
+    // doesn't outlive the test on error.
+    //
+    // This code doesn't work on Windows. It's cleanup code that only runs when
+    // node2 doesn't panic as expected. So it's ok to skip it. See #1781.
+    #[cfg(unix)]
     if node2.is_running() {
         use color_eyre::eyre::eyre;
         return node2
-            .kill_on_error::<(), _>(Err(eyre!("conflicted node2 did not panic")))
+            .kill_on_error::<(), _>(Err(eyre!(
+                "conflicted node2 was still running, but the test expected a panic"
+            )))
             .context_from(&output1)
             .map(|_| ());
     }