test: complete coverage of lib/child_process.js

This commit adds a test which brings coverage of
lib/child_process.js to 100%. It adds coverage for the call to
uv.errname() in execFile()'s exithandler() function.

PR-URL: nodejs/node#12367
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-child-process-execfile.js

@@ -1,9 +1,9 @@
 'use strict';
-
 const common = require('../common');
 const assert = require('assert');
 const execFile = require('child_process').execFile;
 const path = require('path');
+const uv = process.binding('uv');
 
 const fixture = path.join(common.fixturesDir, 'exit.js');
 
@@ -19,3 +19,22 @@ const fixture = path.join(common.fixturesDir, 'exit.js');
     })
   );
 }
+
+{
+  // Verify that negative exit codes can be translated to UV error names.
+  const errorString = `Error: Command failed: ${process.execPath}`;
+  const code = -1;
+  const callback = common.mustCall((err, stdout, stderr) => {
+    assert.strictEqual(err.toString().trim(), errorString);
+    assert.strictEqual(err.code, uv.errname(code));
+    assert.strictEqual(err.killed, true);
+    assert.strictEqual(err.signal, null);
+    assert.strictEqual(err.cmd, process.execPath);
+    assert.strictEqual(stdout.trim(), '');
+    assert.strictEqual(stderr.trim(), '');
+  });
+  const child = execFile(process.execPath, callback);
+
+  child.kill();
+  child.emit('close', code, null);
+}