test: mark test_threadsafe_function/test as flaky

The test fails consistently on windows-fanned with vs2017.
mark it as flaky while the issue is being progressed, and
to keep CI green / amber.

Ref: https://github.com/nodejs/node/issues/23621
PR-URL: https://github.com/nodejs/node/pull/24714


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>