$ cd <openjdk_repository>/test/jdk
$ JTREG_HOME=<jtreg build>
$ JDK_HOME=<full JDK build>
$ TINY_JDK_HOME=`pwd`/tiny_jdk
$ rm -rf $TINY_JDK_HOME
$ $JDK_HOME/bin/jlink --add-modules java.base --output $TINY_JDK_HOME
$ cat > Test.java <<EOT
/*
 * @test
 */
public class Test {
public static void main(String[] argv) {
}
}
EOT
$ $JTREG_HOME/bin/jtreg -testjdk:$TINY_JDK_HOME -compilejdk:$JDK_HOME -noreport -v1 `pwd`/Test.java

I receive this output:
java.lang.NullPointerException
Error: failed to get JDK properties for .../test/jdk/tiny_jdk/bin/java ; exit code 1

Same commands work fine in a directory with an empty TEST.ROOT, which is why I am guessing this happens because "requires.properties".