RFR: JDK-8266942: gtest/GTestWrapper.java os.iso8601_time_vm failed

[Thomas Stuefe]

My test only worked on half the globe, since the timezone stamp comparison was wrong (always compared for '+').

Fixed that. Also made the UTC-variant-tests more strict by requiring the timezone delta to be 0000.

Tested manually with various time zones.

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Commit messages:
 - fix test

Changes: https://git.openjdk.java.net/jdk/pull/4013/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4013&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266942
  Stats: 6 lines in 1 file changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4013.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4013/head:pull/4013

PR: https://git.openjdk.java.net/jdk/pull/4013