RFR: 8276994: java/nio/channels/Channels/TransferTo.java leaves multi-GB files in /tmp

[Markus KARG]

This PR proposes to cleanup *all* temp files of the `TransferTo` test, not just the 2GB files. Also it separates the actual test logic from the cleanup logic, so the test cases are simple to understand again as they are not interwoven with cleanup code anymore.

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Commit messages:
 - Cleaning up temp files

Changes: https://git.openjdk.java.net/jdk/pull/6379/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6379&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276994
  Stats: 67 lines in 1 file changed: 37 ins; 14 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6379.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6379/head:pull/6379

PR: https://git.openjdk.java.net/jdk/pull/6379