make: use low-scrypt to speed up unit-race

This predominately affects the lnwallet unit tests which are the bulk of
the execution time.

lnwallet/btcwallet/btcwallet_rpctest.go

@@ -1,4 +1,4 @@
-// +build rpctest
+// +build rpctest lowscrypt
 
 package btcwallet
 

make/testing_flags.mk

@@ -86,7 +86,7 @@ UNIT_TARGETED ?= no
 # targeted case. Otherwise, default to running all tests.
 ifeq ($(UNIT_TARGETED), yes)
 UNIT := $(GOTEST) -tags="$(DEV_TAGS) $(LOG_TAGS)" $(TEST_FLAGS) $(UNITPKG)
-UNIT_RACE := $(GOTEST) -tags="$(DEV_TAGS) $(LOG_TAGS)" $(TEST_FLAGS) -race $(UNITPKG)
+UNIT_RACE := $(GOTEST) -tags="$(DEV_TAGS) $(LOG_TAGS) lowscrypt" $(TEST_FLAGS) -race $(UNITPKG)
 endif
 
 ifeq ($(UNIT_TARGETED), no)