itest: shuffle test cases to even out blocks mined in tranches

This commit adds a new flag to shuffle all the test cases before running
them so tests which require lots of blocks to be mined are less likely
to be run in the same tranch.

The other benefit is this approach provides a more efficient way to
figure which tests are broken since all the differnet backends are
running different tranches in their builds, we can identify more failed
tests in one push.

scripts/itest_parallel.sh

@@ -3,9 +3,10 @@
 # Get all the variables.
 PROCESSES=$1
 TRANCHES=$2
+SHUFFLE_SEED=$3
 
-# Here we also shift 2 times and get the rest of our flags to pass on in $@.
-shift 2
+# Here we also shift 3 times and get the rest of our flags to pass on in $@.
+shift 3
 
 # Create a variable to hold the final exit code.
 exit_code=0
@@ -13,7 +14,7 @@ exit_code=0
 # Run commands using xargs in parallel and capture their PIDs
 pids=()
 for ((i=0; i<PROCESSES; i++)); do 
-    scripts/itest_part.sh $i $TRANCHES $@ &
+    scripts/itest_part.sh $i $TRANCHES $SHUFFLE_SEED $@ &
     pids+=($!)
 done