build: 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

PR-URL: https://github.com/joyent/node/pull/25686


Reviewed-By: Julien Gilli <julien.gilli@joyent.com>