test: fix test-vm-sigint flakiness

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

Fixes: https://github.com/nodejs/node/issues/7767
PR-URL: https://github.com/nodejs/node/pull/7854
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-vm-sigint.js

@@ -25,14 +25,14 @@ if (process.argv[2] === 'child') {
 }
 
 process.env.REPL_TEST_PPID = process.pid;
-const child = spawn(process.execPath, [ __filename, 'child' ], {
-  stdio: [null, 'pipe', 'inherit']
-});
-
 process.on('SIGUSR2', common.mustCall(() => {
   process.kill(child.pid, 'SIGINT');
 }));
 
+const child = spawn(process.execPath, [ __filename, 'child' ], {
+  stdio: [null, 'pipe', 'inherit']
+});
+
 child.on('close', common.mustCall((code, signal) => {
   assert.strictEqual(signal, null);
   assert.strictEqual(code, 0);