RFR: 8290180: Convert com/sun/management/UnixOperatingSystemMXBean/GetOpenFileDescriptorCount.sh to jtreg java version [v3]

[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").

Bill Huang has updated the pull request incrementally with one additional commit since the last revision:

  Cleaned up tests and changed OS restriction to not windows.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/9503/files
  - new: https://git.openjdk.org/jdk/pull/9503/files/13d99c69..76c46d03

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=9503&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=9503&range=01-02

  Stats: 53 lines in 2 files changed: 1 ins; 34 del; 18 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