test: fix flaky test-vm-sigint-existing-handler

Set the `SIGUSR2` handler before spawning the child process to make sure
the signal is always handled.

Ref: nodejs#7854
Fixes: nodejs#7981

test/parallel/test-vm-sigint-existing-handler.js

@@ -61,16 +61,17 @@ if (process.argv[2] === 'child') {
 }
 
 process.env.REPL_TEST_PPID = process.pid;
-const child = spawn(process.execPath, [ __filename, 'child' ], {
-  stdio: [null, 'inherit', 'inherit']
-});
 
 process.on('SIGUSR2', common.mustCall(() => {
   // First kill() breaks the while(true) loop, second one invokes the real
   // signal handlers.
   process.kill(child.pid, 'SIGINT');
 }, 3));
 
+const child = spawn(process.execPath, [ __filename, 'child' ], {
+  stdio: [null, 'inherit', 'inherit']
+});
+
 child.on('close', function(code, signal) {
   assert.strictEqual(signal, null);
   assert.strictEqual(code, 0);