RFR: 8284866: Add test to JDK-8273056

[Raffaello Giulietti]

Hello,
this is a followup of JDK-8273056 to add a test to check that Random.nextExponential() produces non-negative results.

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Commit messages:
 - 8284866: Add test to JDK-8273056

Changes: https://git.openjdk.java.net/jdk/pull/8243/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8243&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8284866
  Stats: 35 lines in 1 file changed: 35 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8243.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8243/head:pull/8243

PR: https://git.openjdk.java.net/jdk/pull/8243