test: check that pending warning is emitted once

Code for the new --pending-deprecation flag contains logic to make sure
the deprecation warning is emitted only once. However, this was not
being tested. Add test coverage for this situation.

PR-URL: #12527
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-buffer-pending-deprecation.js

@@ -12,4 +12,10 @@ const bufferWarning = 'The Buffer() and new Buffer() constructors are not ' +
 
 common.expectWarning('DeprecationWarning', bufferWarning);
 
+// This is used to make sure that a warning is only emitted once even though
+// `new Buffer()` is called twice.
+process.on('warning', common.mustCall());
+
+new Buffer(10);
+
 new Buffer(10);