RFR: 8276994: java/nio/channels/Channels/TransferTo.java leaves multi-GB files in /tmp [v3]

[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.

Markus KARG has updated the pull request incrementally with one additional commit since the last revision:

  Creating temp files in current working directory
  
  Signed-off-by: Markus Karg <markus at headcrashing.eu>

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6379/files
  - new: https://git.openjdk.java.net/jdk/pull/6379/files/7b9ac28a..b3719b75

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6379&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6379&range=01-02

  Stats: 6 lines in 1 file changed: 2 ins; 0 del; 4 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