The TestDiagsLineCaret test was added in JDK-8267126. I noticed that the test fails locally on my machine, but seems to pass in the mainline CI.

One difference I found was the test mode: CI seems to use "agentvm", while I use the default mode. So when I run the test like this, it always fails:

$ jtreg -testjdk:build/macosx-x64/images/jdk/ test/langtools/jdk/javadoc/doclet/testDiagsLineCaret/TestDiagsLineCaret.java
Test results: failed: 1

and passes when run like this:

$ jtreg -agentvm -testjdk:build/macosx-x64/images/jdk/ test/langtools/jdk/javadoc/doclet/testDiagsLineCaret/TestDiagsLineCaret.java
Test results: passed: 1