RFR: 8294437: java/nio/channels/FileChannel tests slow on Windows [v3]

[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)

Daniel Jeliński has updated the pull request incrementally with four additional commits since the last revision:

 - Add explicit build tags
 - Move createSparseTempFile closer to FileChannel
 - add randomness
 - Import static StandardOpenOption

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/10464/files
  - new: https://git.openjdk.org/jdk/pull/10464/files/614d0e54..08829f02

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10464&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10464&range=01-02

  Stats: 96 lines in 6 files changed: 52 ins; 25 del; 19 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