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: https://github.com/nodejs/node/pull/7854
Fixes: https://github.com/nodejs/node/issues/7981
PR-URL: https://github.com/nodejs/node/pull/7982
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

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

@@ -61,16 +61,19 @@ if (process.argv[2] === 'child') {
 }
 
 process.env.REPL_TEST_PPID = process.pid;
-const child = spawn(process.execPath, [ __filename, 'child' ], {
-  stdio: [null, 'inherit', 'inherit']
-});
 
+// Set the `SIGUSR2` handler before spawning the child process to make sure
+// the signal is always handled.
 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);

test/parallel/test-vm-sigint.js

@@ -25,6 +25,9 @@ if (process.argv[2] === 'child') {
 }
 
 process.env.REPL_TEST_PPID = process.pid;
+
+// Set the `SIGUSR2` handler before spawning the child process to make sure
+// the signal is always handled.
 process.on('SIGUSR2', common.mustCall(() => {
   process.kill(child.pid, 'SIGINT');
 }));