RFR: add simple test workflow

[Igor Ignatyev]

Hi all,

could you please review this small patch that adds a simple github action workflow to build and test `jtreg` on `linux-x64`?

Thanks,
-- Igor

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Commit messages:
 - add simple test workflow

Changes: https://git.openjdk.java.net/jtreg/pull/16/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtreg&pr=16&range=00
  Stats: 40 lines in 1 file changed: 40 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jtreg/pull/16.diff
  Fetch: git fetch https://git.openjdk.java.net/jtreg pull/16/head:pull/16

PR: https://git.openjdk.java.net/jtreg/pull/16