test: fix error in test-cluster-worker-death.js

Replaced calls to assert.equal with assert.strictEqual in order
to fix the following error:
"Please use assert.strictEqual() instead of assert.strictEqual()"

PR-URL: nodejs#9981
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Bruce Lai

test/parallel/test-cluster-worker-death.js

@@ -8,10 +8,10 @@ if (!cluster.isMaster) {
 } else {
   var worker = cluster.fork();
   worker.on('exit', common.mustCall(function(exitCode, signalCode) {
-    assert.equal(exitCode, 42);
-    assert.equal(signalCode, null);
+    assert.strictEqual(exitCode, 42);
+    assert.strictEqual(signalCode, null);
   }));
   cluster.on('exit', common.mustCall(function(worker_) {
-    assert.equal(worker_, worker);
+    assert.strictEqual(worker_, worker);
   }));
 }