RFR: 8290180: Convert com/sun/management/UnixOperatingSystemMXBean/GetOpenFileDescriptorCount.sh to jtreg java version

[Bill Huang]

This is a task to convert shell tests to Java tests. The shell scripts to be removed are responsible for compiling Java files and running the tests on Linux systems. This functionality can be achieved with JTReg tag @requires (os.family=="linux").

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Commit messages:
 - Updated copyright header.
 - Converted FileDescriptorCount shell tests to java tests.

Changes: https://git.openjdk.org/jdk/pull/9503/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9503&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290180
  Stats: 130 lines in 4 files changed: 7 ins; 119 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/9503.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9503/head:pull/9503

PR: https://git.openjdk.org/jdk/pull/9503