RFR: 7903763: [jtreg] grep: /proc/version: No such file or directory

[SendaoYan]

Hi all,
In rpmbuild mock enviroment, there is not `/proc/version` file, `jtreg -version` command will print `grep: /proc/version: No such file or directory` message.
This PR add check the file `/proc/version` exists or not before execute command `grep /proc/version`
The change has been verified locally, no risk.

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Commit messages:
 - 7903763: [jtreg] grep: /proc/version: No such file or directory

Changes: https://git.openjdk.org/jtreg/pull/210/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=210&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903763
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/210.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/210/head:pull/210

PR: https://git.openjdk.org/jtreg/pull/210