make+itest: add tranche index to test name

To make it easier to see what tranche a failed test was running in, we
add the tranche index to the test name. This corresponds to the number
in the .logs-tranche<index> folder name so the logs can be found much
quicker.

make/testing_flags.mk

@@ -46,7 +46,7 @@ endif
 
 # Define the integration test.run filter if the icase argument was provided.
 ifneq ($(icase),)
-TEST_FLAGS += -test.run="TestLightningNetworkDaemon/.*-of-.*/.*/$(icase)"
+TEST_FLAGS += -test.run="TestLightningNetworkDaemon/tranche.*/.*-of-.*/.*/$(icase)"
 endif
 
 # Run itests with specified db backend.