RFR: 8303001: Add test for re-entrant upcalls

[Jorn Vernee]

Add a test that tests re-entrant upcalls, which is a use-case that is uncovered by testing until now.

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Commit messages:
 - add reentrant upcalls test

Changes: https://git.openjdk.org/jdk/pull/12927/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12927&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303001
  Stats: 122 lines in 2 files changed: 122 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12927.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12927/head:pull/12927

PR: https://git.openjdk.org/jdk/pull/12927