add a system property to allow testing of suspicious stuff [LUCENE-4630]

there are times when people want to add assumptions in test to prevent confusing/false failures in certain situations (eg: known bugs in JVM X, known incompatibilities between lucene feature Z and filesystem Y, etc...)

By default we want these situations to be "skiped" in tests with clear messages so that it's clear to end users trying out releases that these tests can't be run for specific sitautions.

But at the same time we need a way for developers to be able to try running these tests anyway so we know if/when the underliyng problem is resolved.

i propose we add a "tests.suspicious.shit" system property, which defaults to "false" in the javacode, but can be set at runtime to "true"

assumptions about things like incompatibilities with OSs, JVM vendors, JVM versions, filesystems, etc.. can all be dependent on this system propery.

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Migrated from [LUCENE-4630](https://issues.apache.org/jira/browse/LUCENE-4630) by Chris M. Hostetter (@hossman), updated May 09 2016
Linked issues:
- #5696

No source files, tests, or entry points are named. Start by locating the shared test-assumption or test-configuration code and inspect linked issue #5696 for context. Done means suspicious assumptions remain skipped by default with clear messages, while developers can enable them at runtime with the proposed system property.

Written by the indexing model from the issue text.