test: fix test-child-process-flush-stdio on win

This test cases tries to use the V6 "shell:true" option
on child_process.spawn that isn't in V4, therefore the
test failes when there is no "echo.exe" on Windows systems.
This fix implements the same functionality (cmd/c echo)
that the shell:true option would have given.

test/parallel/test-child-process-flush-stdio.js

@@ -4,10 +4,14 @@ const common = require('../common');
 const assert = require('assert');
 
 // Windows' `echo` command is a built-in shell command and not an external
-// executable like on *nix
-const opts = { shell: common.isWindows };
-
-const p = cp.spawn('echo', [], opts);
+// executable like on *nix. The V4 API does not have the "shell" option to
+// spawn that we use in V4 and later so we can't use that here.
+var p;
+if (common.isWindows) {
+  p = cp.spawn('cmd.exe', ['/c', 'echo'], {});
+} else {
+  p = cp.spawn('echo', [], {});
+}
 
 p.on('close', common.mustCall(function(code, signal) {
   assert.strictEqual(code, 0);
@@ -19,7 +23,12 @@ p.stdout.read();
 
 function spawnWithReadable() {
   const buffer = [];
-  const p = cp.spawn('echo', ['123'], opts);
+  var p;
+  if (common.isWindows) {
+    p = cp.spawn('cmd.exe', ['/c', 'echo', '123'], {});
+  } else {
+    p = cp.spawn('echo', ['123'], {});
+  }
   p.on('close', common.mustCall(function(code, signal) {
     assert.strictEqual(code, 0);
     assert.strictEqual(signal, null);