multi: add new build tag integration

This commit adds a new build tag `integration` and removes the old tag
`rpctest` for clarity. Multiple unnecessary usages of `build !rpctest`
is also removed.

make/testing_flags.mk

@@ -115,4 +115,4 @@ backend = btcd
 endif
 
 # Construct the integration test command with the added build flags.
-ITEST_TAGS := $(DEV_TAGS) $(RPC_TAGS) rpctest $(backend)
+ITEST_TAGS := $(DEV_TAGS) $(RPC_TAGS) integration $(backend)