RFR: Touch up gradle build

[Jorn Vernee]

I'm setting up the jextract repo on a new machine, and ran into a couple of small build issues that this PR addresses:

1.) intellij will try to collect all tasks defined by the gradle build, but this is currently failing if you don't have the jacoco agent configured, since that's needed to 'configure' one of the test tasks. The fix I added is to only lazily register the test task for coverage instead.

2.) If you want to debug a jtreg test through Intellij, it's nice to be able to build the test image and native test libraries as a pre-run step. I've added a new task called `testDeps` which does both of these things, and can be easily referenced from a run configuration.

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Commit messages:
 - Touch up build file

Changes: https://git.openjdk.org/jextract/pull/263/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=263&range=00
  Stats: 19 lines in 1 file changed: 9 ins; 5 del; 5 mod
  Patch: https://git.openjdk.org/jextract/pull/263.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/263/head:pull/263

PR: https://git.openjdk.org/jextract/pull/263