RFR: 7902698: Collision of zero timestamp as a special value with use in filesystems

[Liam Miller-Cushon]

This fixes a bug when the timestamp of a test file is set to zero, since jtharness currently uses zero as a special value. See discussion in [CODETOOLS-7902698](https://bugs.openjdk.java.net/browse/CODETOOLS-7902698).

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Commit messages:
 - 7902698: Collision of zero timestamp as a special value with use in filesystems

Changes: https://git.openjdk.java.net/jtharness/pull/22/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jtharness&pr=22&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902698
  Stats: 10 lines in 3 files changed: 0 ins; 0 del; 10 mod
  Patch: https://git.openjdk.java.net/jtharness/pull/22.diff
  Fetch: git fetch https://git.openjdk.java.net/jtharness pull/22/head:pull/22

PR: https://git.openjdk.java.net/jtharness/pull/22