If I try to build jtreg 5.0-b01 in the obvious way using

 $ (set -eux; rm -rf /tmp/t8; mkdir /tmp/t8; cd /tmp/t8; hg clone -r jtreg5.0-b01 http://hg.openjdk.java.net/code-tools/jtreg jtreg; cd jtreg; bash make/build-all.sh ~/jdk/jdk8; cd build/images/jtreg; cat release; bin/jtreg -version)

I see ... off-by-one version

echo "JTREG_VERSION=4.2 b16" > /tmp/t8/jtreg/build/images/jtreg/release
...
+/bin/zsh:79> cat release
JTREG_VERSION=4.2 b16
BUILD_DATE=March 02, 2020
SOURCE=jtreg:03c4dff80b0f
+/bin/zsh:79> bin/jtreg -version
jtreg, version 4.2 dev b16
...