Fix unit test to use python3 command

unit test jobs staretd to run on ubuntu-focal now
and failing for using 'python' command.

Change-Id: Ie002faf4c96ac7f207207a481c057b8df0289e6c
diff --git a/tests/unittest.sh b/tests/unittest.sh
index 3703ece..fced2ab 100644
--- a/tests/unittest.sh
+++ b/tests/unittest.sh
@@ -17,6 +17,8 @@
 PASS=0
 FAILED_FUNCS=""
 
+export PYTHON=$(which python3 2>/dev/null)
+
 # pass a test, printing out MSG
 #  usage: passed message
 function passed {