git: openjdk/loom: fibers: Test vrthed in premain() added.

[duke]

Changeset: 836702d1
Author:    lmesnik <leonid.mesnik at oracle.com>
Date:      2020-12-15 13:15:04 +0000
URL:       https://git.openjdk.java.net/loom/commit/836702d1

Test vrthed in premain() added.

+ test/hotspot/jtreg/serviceability/jvmti/vthread/premain/AgentWithVThread.java
+ test/hotspot/jtreg/serviceability/jvmti/vthread/premain/AgentWithVThreadTest.java