benchmark: terminate child process on Windows

test-benchmark-child-process failures reveal that
child-process-exec-stdout benchmark sometimes leaves around a stray
yes.exe process. Add code to terminate the process.

Refs: nodejs#12560

Author: Trott

benchmark/child_process/child-process-exec-stdout.js

@@ -10,7 +10,8 @@ const bench = common.createBenchmark(main, {
   dur: [5]
 });
 
-const exec = require('child_process').exec;
+const child_process = require('child_process');
+const exec = child_process.exec;
 function main(conf) {
   bench.start();
 
@@ -29,7 +30,12 @@ function main(conf) {
   });
 
   setTimeout(function() {
-    child.kill();
     bench.end(bytes);
+    if (process.platform === 'win32') {
+      // Sometimes there's a yes.exe process left hanging around on Windows...
+      child_process.execSync(`taskkill /f /t /pid ${child.pid}`);
+    } else {
+      child.kill();
+    }
   }, dur * 1000);
 }