In jtreg -help, -jdk:<java.home> and testjdk:<java.home> are in the same documentation lines, and thus would appear to do the same thing, but they are not the same when it comes to calculating $JTREG_JAVA:

    -jdk:<java.home> | -testjdk:<java.home>
                    Run all tests using specified JDK. For example,
                    -jdk:/usr/local/java/jdk1.5/solaris-sparc

https://openjdk.org/jtreg/runtests.html says:

    If you want to explicitly specify the JDK used to run jtreg, set the JT_JAVA
    environment variable before running jtreg. Otherwise, leave it unset, and
    jtreg will default to using the same version of JDK to run jtreg as is used to
    execute the tests.

But jtreg doesn't actually look at -testjdk, just -jdk:

     # Look for -jdk option as possible default to run jtreg
    ...deleted...
    for i in "$@" ; do
        case $i in
        -jdk:* ) jdk="`echo $i | sed -e 's/^-jdk://'`" ;;
        esac
    ...deleted...
    done

    # Determine java for jtreg, from JTREG_JAVA, JAVA_HOME, -jdk, java
    JTREG_JAVA=${JTREG_JAVA:-$JT_JAVA} # allow for old version of name
    if [ -n "$JTREG_JAVA" ]; then
        if [ -d "$JTREG_JAVA" ]; then
            JTREG_JAVA="$JTREG_JAVA/bin/java"
        fi
    elif [ -n "$JAVA_HOME" ]; then
        JTREG_JAVA="$JAVA_HOME/bin/java"
    elif [ -n "$jdk" ]; then
        JTREG_JAVA="$jdk/bin/java"
    else
        JTREG_JAVA=java
    fi

    if [ ! -e "$JTREG_JAVA" ]; then
        echo "No java executable at $JTREG_JAVA"
        exit 1;
    fi

Either fix the docs to make this clear, or fix jtreg to accept either.