test: support flaky tests in test-ci

Adding support for specifying flaky test mode to
the test runner:
- via an environment variable FLAKY_TESTS for Makefile
- via an argument ignore-flaky for vcbuild.bat

Ported from https://github.com/joyent/node/commit/2d2494cf140c38327218a087593ff2177a9d0ec9

PR-URL: https://github.com/nodejs/node/pull/2424


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: João Reis <reis@janeasystems.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>