RFR: 7903592: Add test source set

[Jorn Vernee]

Add a source set for the test source directories in gradle. And add the jtreg jars as dependencies of test compilation. Intellij will then automatically mark the directories as test source directories, and add the jtreg jar files to the class path of the tests (so that e.g. the `@Test` annotation and other testng imports are recognized)

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Commit messages:
 - add test directory source sets

Changes: https://git.openjdk.org/jextract/pull/143/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=143&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903592
  Stats: 15 lines in 1 file changed: 15 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/143.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/143/head:pull/143

PR: https://git.openjdk.org/jextract/pull/143