RFR: 8295798: (ch) Test java/nio/channels/Channels/ReadXBytes.java is very slow on Windows [v2]

[Daniel Jeliński]

> Please review this PR that reduces the number of bytes written by ReadXBytes test on machines supporting sparse files.
> 
> Before this change the `createFileWithRandomContent` function would usually write either the last byte only, or the entire file (`long p` was usually set to length - 1). Now the function only writes the first and the last 32kB, and the remaining bytes are initialized in platform-defined manner.
> 
> Before this change the test took between 1-10 minutes to complete on our Windows CI machines.
> After this change the test finishes in less than 1 minute.

Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10834/files
  - new: https://git.openjdk.org/jdk/pull/10834/files/70f4b4bf..f5229267

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10834&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10834&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/10834.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10834/head:pull/10834

PR: https://git.openjdk.org/jdk/pull/10834