RFR: 8305596: (fc) Two java/nio/channels tests fail after JDK-8303260

[Brian Burkhalter]

Update the tests to conform to the specification change in  [JDK-8303260](https://bugs.openjdk.org/browse/JDK-8303260) which allows writing to a position beyond the channel's size in `FileChannel::transferFrom`.

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Commit messages:
 - 8305596: (fc) Two java/nio/channels tests fail after JDK-8303260

Changes: https://git.openjdk.org/jdk/pull/13342/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13342&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305596
  Stats: 14 lines in 3 files changed: 4 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13342.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13342/head:pull/13342

PR: https://git.openjdk.org/jdk/pull/13342