RFR: 8255487: Mark SandboxAppTest unstable on Windows

[Ajit Ghaisas]

On Tue, 27 Oct 2020 23:11:44 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> This test is somewhat unstable on Windows. Even after the test timeout was increased to 25 seconds, it still times out on some Windows machines.
> 
> The proposed fix is to mark this test unstable on Windows so our nightly build will not execute it and get frequent failures. It can still be run by passing `-PUNSTABLE_TEST=true` to gradle. For example:
> 
> gradle --info -PUNSTABLE_TEST=true -PFULL_TEST=true cleanTest :systemTests:test --tests SandboxAppTest

Marked as reviewed by aghaisas (Reviewer).

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PR: https://git.openjdk.java.net/jfx/pull/340