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.

make/testing_flags.mk

@@ -10,6 +10,7 @@ COVER_PKG = $$(go list -deps -tags="$(DEV_TAGS)" ./... | grep '$(PKG)' | grep -v
 NUM_ITEST_TRANCHES = 4
 ITEST_PARALLELISM = $(NUM_ITEST_TRANCHES)
 POSTGRES_START_DELAY = 5
+SHUFFLE_SEED = 0
 
 # If rpc option is set also add all extra RPC tags to DEV_TAGS
 ifneq ($(with-rpc),)
@@ -27,6 +28,11 @@ ifneq ($(parallel),)
 ITEST_PARALLELISM = $(parallel)
 endif
 
+# Set the seed for shuffling the test cases.
+ifneq ($(shuffleseed),)
+SHUFFLE_SEED = $(shuffleseed)
+endif
+
 # Windows needs to append a .exe suffix to all executable files, otherwise it
 # won't run them.
 ifneq ($(windows),)