[foreign-jextract] RFR: Tweak JextractToolRunner to use file helper routines in FileUtils library

[Maurizio Cimadamore]

This is an attempt to fix some of the jextract tests that are failing in our build infra; I looked over the tests and I
couldn't find anything obviously wrong (such as streams left open and such). But speaking with Alan, he raised the
possibility that the failures are a result of a known bad interaction with the AV solution installed in some of the
Windows test machines. To workaround this, we have an helper library called FileUtils which has a deleteFileWithRetry
method, which is meant to address these issues. I think it could be worth trying whether this fixes our issues before
investigating further - in any case, at the very least this patch improves the code by reusing sharing helper
functionalities already available in the test folder.

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Commit messages:
 - Tweak JextractToolRunner to use file helper routines in FileUtils library

Changes: https://git.openjdk.java.net/panama-foreign/pull/100/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/100/webrev.00
  Stats: 24 lines in 10 files changed: 10 ins; 12 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/100.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/100/head:pull/100

PR: https://git.openjdk.java.net/panama-foreign/pull/100