test: fix flaky test-repl-sigint-nested-eval

There is a race condition where process.kill can be sent before the
target is ready to receive the signal.

Fixes: nodejs#41123

test/parallel/parallel.status

@@ -5,8 +5,6 @@ prefix parallel
 # sample-test                        : PASS,FLAKY
 
 [true] # This section applies to all platforms
-# https://github.com/nodejs/node/issues/41123
-test-repl-sigint-nested-eval: PASS, FLAKY
 # https://github.com/nodejs/node/issues/43084
 test-worker-http2-stream-terminate: PASS, FLAKY
 

test/parallel/test-repl-sigint-nested-eval.js

@@ -12,7 +12,7 @@ const spawn = require('child_process').spawn;
 
 process.env.REPL_TEST_PPID = process.pid;
 const child = spawn(process.execPath, [ '-i' ], {
-  stdio: [null, null, 2]
+  stdio: [null, null, 2, 'ipc']
 });
 
 let stdout = '';
@@ -22,17 +22,19 @@ child.stdout.on('data', function(c) {
 });
 
 child.stdout.once('data', common.mustCall(() => {
-  process.on('SIGUSR2', common.mustCall(() => {
+  child.on('message', common.mustCall((msg) => {
+    assert.strictEqual(msg, 'repl is busy');
     process.kill(child.pid, 'SIGINT');
     child.stdout.once('data', common.mustCall(() => {
       // Make sure REPL still works.
       child.stdin.end('"foobar"\n');
     }));
   }));
 
-  child.stdin.write('process.kill(+process.env.REPL_TEST_PPID, "SIGUSR2");' +
-                    'vm.runInThisContext("while(true){}", ' +
-                    '{ breakOnSigint: true });\n');
+  child.stdin.write(
+    'vm.runInThisContext("process.send(\'repl is busy\'); while(true){}", ' +
+    '{ breakOnSigint: true });\n'
+  );
 }));
 
 child.on('close', function(code) {