test: add test for attempted multiple IPC channels

This error was previously not covered. This commit adds coverage.

PR-URL: nodejs#8159
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

test/parallel/test-child-process-stdio.js

@@ -16,4 +16,8 @@ assert.equal(child.stderr, null);
 
 options = {stdio: 'ignore'};
 child = common.spawnSyncCat(options);
-assert.deepEqual(options, {stdio: 'ignore'});
+assert.deepStrictEqual(options, {stdio: 'ignore'});
+
+assert.throws(() => {
+  common.spawnPwd({stdio: ['pipe', 'pipe', 'pipe', 'ipc', 'ipc']});
+}, /^Error: Child process can have only one IPC pipe$/);