TxnTSTest: refactor tests from FlatSpec to FunSpec

Signed-off-by: 35V LG84 <35vlg84-x4e6b92@e257.fi>
2 jobs for txnts-shard in 4 minutes and 53 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #150116096
test·jdk11

00:04:53

passed #150116097
test·jdk8

00:04:41

99.73%