RFR: 8345134: Test sun/security/tools/jarsigner/ConciseJarsigner.java failed: unable to find valid certification path to requested target

[Mikhail Yankelevich]

Changed the gencert, so the certificate is now initialised to be with a start time of -1M. This prevents a potential error with the certificate not yet being valid. 
Similar solution to [this pull request](https://github.com/openjdk/jdk/pull/22592)
Have tested the test 6000 times for stability and all have passed.

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Commit messages:
 - minor: date change
 - JDK-8345134: Test sun/security/tools/jarsigner/ConciseJarsigner.java failed: unable to find valid certification path to requested target

Changes: https://git.openjdk.org/jdk/pull/23001/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23001&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345134
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23001.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23001/head:pull/23001

PR: https://git.openjdk.org/jdk/pull/23001