test: add test for child_process.execFile()

While `child_process.execFile()` gets called in places in the test
suite, there are no explicit test for it and there are parts of the
implementation that are not covered by tests. This adds a minimal test
that increases (but does not complete) coverage for the implementation.

PR-URL: nodejs/node#11929
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

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

@@ -0,0 +1,21 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const execFile = require('child_process').execFile;
+const path = require('path');
+
+const fixture = path.join(common.fixturesDir, 'exit.js');
+
+{
+  execFile(
+    process.execPath,
+    [fixture, 42],
+    common.mustCall((e) => {
+      // Check that arguments are included in message
+      assert.strictEqual(e.message.trim(),
+                         `Command failed: ${process.execPath} ${fixture} 42`);
+      assert.strictEqual(e.code, 42);
+    })
+  );
+}