Thread dump is never collected when a specific test timed out.

It is http://hg.openjdk.java.net/jdk9/dev/jdk/file/82ed25c3cea9/test/java/net/httpclient/SmokeTest.java. The test timed out sometimes, especially on Windows, because of a product issue (JDK-8178699)

This can be reproduced with recent jdk 9 or jdk 10 promotion builds no matter how the test executed. Observed in jdk promotion test run, CI jobs or even simple local run. You can see JDK-8178699 for history logs.

Reproduced the problem with jtreg 4.2 b07 fcs b07 by a simple command locally:
jtreg/bin/jtreg -jdk:$JDK9_b174 -nr 9-dev/jdk/test/java/net/httpclient/SmokeTest.java

There will be only "Timeout signalled after 600 seconds " in the jtr and no thread dump.