RFR: 7902698: Collision of zero timestamp as a special value with use in filesystems [v2]

[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).

Liam Miller-Cushon has updated the pull request incrementally with one additional commit since the last revision:

  Use OptionalLong instead of -1L

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Changes:
  - all: https://git.openjdk.java.net/jtharness/pull/22/files
  - new: https://git.openjdk.java.net/jtharness/pull/22/files/b56f7172..c988435a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jtharness&pr=22&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jtharness&pr=22&range=00-01

  Stats: 32 lines in 3 files changed: 5 ins; 0 del; 27 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