RFR: 8319174: Enhance robustness of some j.m.BigInteger constructors

[Raffaello Giulietti]

Enhance `java.math.BigInteger` constructors taking a `byte[]` argument by improving guarantees of internal invariants.

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Commit messages:
 - Added @bug to test.
 - Refactored test to JUnit.
 - 8319174: Enhance robustness of some j.m.BigInteger constructors
 - Merge branch 'master'
 - Merge branch 'master' into bigint
 - Enhance BigInteger constructors taking byte[]

Changes: https://git.openjdk.org/jdk/pull/16449/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16449&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319174
  Stats: 327 lines in 3 files changed: 255 ins; 18 del; 54 mod
  Patch: https://git.openjdk.org/jdk/pull/16449.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16449/head:pull/16449

PR: https://git.openjdk.org/jdk/pull/16449