The following tests use core reflection to set the Thread.Builder scheduler:

test/jdk/java/io/ByteArrayOutputStream/WriteToReleasesCarrier.java
test/jdk/jdk/internal/misc/TerminatingThreadLocal/TestTerminatingThreadLocal.java
test/hotspot/jtreg/serviceability/jvmti/vthread/ThreadStateTest/ThreadStateTest.java

These tests have been updated in the loom repo to use jdk.test.lib.thread.VThreadScheduler. That update can be brought to mainline.