arctic runtTests.sh does not "clear" the arctic session between running testcases

[andrew-m-leonard]

arctic runtTests.sh does not "clear" the arctic session between running testcases. This makes it difficult to reload the failure session file in an Arctic preview interaction to see differences manually and create alt-images.

"session clear" needs calling between testcase runs after each "session save"

I think the session does clear correctly on a successful Arctic "test finish" command possibly.