HandlerLoop should be run in othervm, as it sets the system-wide URL protocol handler factory which could inadvertently affect subsequent tests run in the same agentvm. For example, it can affect OpenStream.java:

$ rm -rf JT*
$
$ ~/tools/jtreg/bin/jtreg -agentvm -v:time -conc:1 -esa -ea -jdk:build/macosx-x64/images/jdk test/jdk/java/net/URL/HandlerLoop.java test/jdk/java/net/URL/OpenStream.java
Directory "JTwork" not found: creating
Directory "JTreport" not found: creating
Passed: java/net/URL/HandlerLoop.java
[2019-03-19 20:19:07,599] Agent[0]: stderr: WARNING: An illegal reflective access operation has occurred
[2019-03-19 20:19:07,599] Agent[0]: stderr: WARNING: Illegal reflective access by HandlerLoop$HandlerFactory (file:/Users/chhegar/repos/jdk/jdk/open/JTwork/classes/java/net/URL/HandlerLoop.d/) to constructor sun.net.www.protocol.http.Handler()
[2019-03-19 20:19:07,599] Agent[0]: stderr: WARNING: Please consider reporting this to the maintainers of HandlerLoop$HandlerFactory
[2019-03-19 20:19:07,600] Agent[0]: stderr: WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
[2019-03-19 20:19:07,600] Agent[0]: stderr: WARNING: All illegal access operations will be denied in a future release
  build: 0.956 seconds
  compile: 0.951 seconds
  main: 0.045 seconds
Passed: java/net/URL/OpenStream.java
  build: 0.101 seconds
  compile: 0.101 seconds
  main: 0.027 seconds
Test results: passed: 2
Report written to /Users/chhegar/repos/jdk/jdk/open/JTreport/html/report.html
Results written to /Users/chhegar/repos/jdk/jdk/open/JTwork
$
$ find JTwork/ -name "*.jtr" -exec grep "Agent id" {} \; -print
Agent id: 0
Agent id: 0
JTwork//java/net/URL/OpenStream.jtr
Agent id: 0
Agent id: 0
JTwork//java/net/URL/HandlerLoop.jtr
$
$ grep "Loading handler" JTwork/java/net/URL/HandlerLoop.jtr
Loading handler class: sun.net.www.protocol.file.Handler
$
$ grep "Loading handler" JTwork/java/net/URL/OpenStream.jtr
Loading handler class: sun.net.www.protocol.http.Handler