[jdk7u] RFR: 8285523: Improve test java/io/FileOutputStream/OpenNUL.java

[Olga Mikhaltsova]

I'd like to backport JDK-8285523 to jdk7u.

The patch applied not cleanly due to the directory structure differences:
`test/jdk/java/io/FileOutputStream/OpenNUL.java`-> `jdk/test/java/io/FileOutputStream/OpenNUL.java`

Tested manually as follow:
$JTREG_HOME/bin/jtreg -jdk:$BUILD_HOME /cygdrive/c/projects/jdk7u-fork/jdk/test/java/io/
No regression in the above tests' group.

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Depends on: https://git.openjdk.org/jdk7u/pull/9

Commit messages:
 - Backport f42631e354d4abf7994abd92aa5def6b2ceeab3a

Changes: https://git.openjdk.org/jdk7u/pull/10/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk7u&pr=10&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8285523
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk7u/pull/10.diff
  Fetch: git fetch https://git.openjdk.org/jdk7u pull/10/head:pull/10

PR: https://git.openjdk.org/jdk7u/pull/10