Add a shared test-helper for the integration-tests module

[dfa1]

Background

The unit module shares fixtures via ZstdTestSupport / generators, but the integration-tests module is a separate Maven module and can't see those package-private test classes. So its three test files re-implement the same helpers:

• random(Random, int) and compressible(Random, int) byte generators
• the compression-levels array {min, 1, default, max}

Seen in:

• integration-tests/.../it/ZstdJniInteropTest.java (e.g. payloadsAndLevels() ~L34-45)
• integration-tests/.../it/ZstdInteropExtrasTest.java
• integration-tests/.../it/GoldenCorpusTest.java

Task

Introduce one small package-private helper class in integration-tests/src/test/java/io/github/dfa1/zstd/it/ (e.g. ItTestSupport) holding the shared random/compressible/levels generators, and have the three test classes use it.

Note: keep it inside the integration-tests module — do not try to reuse the unit module's ZstdTestSupport (that would require publishing a test-jar, which is out of scope here).

Acceptance criteria

• The duplicated generators live in one place; the three test files call it.
• ./mvnw -pl integration-tests test stays green.

Pointers

• integration-tests/src/test/java/io/github/dfa1/zstd/it/