test: add known_issues test for #6287

Deleting property in the vm context has no effect
as reported in https://github.com/nodejs/node/issues/6287
The test is moved to the known_issues and will be fixed
with the 5.5 V8 API changes.

PR-URL: https://github.com/nodejs/node/pull/10272


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>