RFR: 8294437: java/nio/channels/FileChannel tests slow on Windows

[Daniel Jeliński]

Please review this test-only change that improves the execution speed of a few FileChannel tests:
- Transfer2GPlus, Transfer4GBFile, and TransferTo6GBFile were modified to use sparse files. Their speed is now consistent across platforms, provided that the platform / filesystem supports sparse files.
- LargeMapTest was rewritten to use sparse files, and to more precisely target the issue it was written to detect. In my tests it still crashed with EXCEPTION_ACCESS_VIOLATION when JDK-8286637 was reverted.
- MapTest: repetition was removed from `testForce` method; I'm not sure what issues it was supposed to catch, but at far as I can tell, it was only triggering timeouts, see [JDK-8289526](https://bugs.openjdk.org/browse/JDK-8289526), [JDK-8224480](https://bugs.openjdk.org/browse/JDK-8224480)

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Commit messages:
 - MapTest: avoid costly repetition
 - Simplify LargeMapTest
 - LargeMapTest: use sparse file
 - Test4g: use sparse files
 - Test6g: use sparse file
 - Test2g: write sparse file

Changes: https://git.openjdk.org/jdk/pull/10464/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10464&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294437
  Stats: 153 lines in 5 files changed: 65 ins; 23 del; 65 mod
  Patch: https://git.openjdk.org/jdk/pull/10464.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10464/head:pull/10464

PR: https://git.openjdk.org/jdk/pull/10464