RFR: 8273487: Zero: Handle "zero" variant in runtime tests

[Aleksey Shipilev]

JDK-8179317 rewritten runtime shell tests to Java. There is a little omission in VM variant selection, which makes Zero fail some of the tier1 tests, like:


$ CONF=linux-x86_64-zero-fastdebug make exploded-test TEST=runtime/StackGap/TestStackGap.java

STDERR:
java.lang.Error: TESTBUG: unsupported vm variant
at jdk.test.lib.Platform.variant(Platform.java:368)
at jdk.test.lib.Platform.jvmLibDir(Platform.java:357)
at jdk.test.lib.process.ProcessTools.addJvmLib(ProcessTools.java:585)
at jdk.test.lib.process.ProcessTools.createNativeTestProcessBuilder(ProcessTools.java:575) 


Additional testing:
 - [ ] Linux x86_64 Zero affected tests (StackGap, StackGuardPages, TestTLS) now run

-------------

Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/5413/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5413&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273487
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5413.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5413/head:pull/5413

PR: https://git.openjdk.java.net/jdk/pull/5413