RFR: 8366298: FDLeakTest sometimes takes minutes to complete on Linux

[Stefan Karlsson]

While investigating [JDK-8260555](https://bugs.openjdk.org/browse/JDK-8260555), which lowers the timeout factor from 4 to 1, we found that FDLeakTest sometimes times out on Linux.

The reason is that the test performs a `fcntl` call for each and every potential file descriptor number. This can be a large number of calls and sometimes results in minutes-long test executions.

I propose that we fix this by limiting the max number of open file descriptors. This lowers the test execution time to about 1 second.

The test has two processes. One that executes the libFDLeaker.c code below as an agent in the test JVM, then it forks into a exeFDLeakTester.c, which reads the property `int max_fd = (int)sysconf(_SC_OPEN_MAX);`. The setting of `RLIMIT_NOFILE` to `100` lowers `max_fd` to `100`. I've verified this on both Linux and on macOS.

I've run the test manually on Linux and macOS and verified that it runs faster. I've also run this through tier1.

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Commit messages:
 - 8366298: FDLeakTest sometimes takes minutes to complete on Linux

Changes: https://git.openjdk.org/jdk/pull/26979/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26979&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366298
  Stats: 32 lines in 1 file changed: 32 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26979.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26979/head:pull/26979

PR: https://git.openjdk.org/jdk/pull/26979