RFR: 8272369: java/io/File/GetXSpace.java failed with "RuntimeException: java.nio.file.NoSuchFileException: /run/user/0"

[Brian Burkhalter]

This change proposes to exclude `/run/user` mounts from the test when run on Linux as they can "disappear" while the test is running.

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Commit messages:
 - 8272369: java/io/File/GetXSpace.java failed with "RuntimeException: java.nio.file.NoSuchFileException: /run/user/0"

Changes: https://git.openjdk.java.net/jdk/pull/5136/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5136&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272369
  Stats: 15 lines in 1 file changed: 7 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5136.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5136/head:pull/5136

PR: https://git.openjdk.java.net/jdk/pull/5136