make+Travis: use EXEC_SUFFIX for Windows, remove explicit goal

To remove the need to have an extra make goal for the Windows itests, we
instead add the flag windows=1 that sets the make variable EXEC_SUFFIX
to properly add the ".exe" suffix to all executable names.

make/testing_flags.mk

@@ -3,6 +3,7 @@ RPC_TAGS = autopilotrpc chainrpc invoicesrpc routerrpc signrpc verrpc walletrpc
 LOG_TAGS =
 TEST_FLAGS =
 ITEST_FLAGS = 
+EXEC_SUFFIX =
 COVER_PKG = $$(go list -deps ./... | grep '$(PKG)' | grep -v lnrpc)
 NUM_ITEST_TRANCHES = 6
 ITEST_PARALLELISM = $(NUM_ITEST_TRANCHES)
@@ -23,6 +24,12 @@ ifneq ($(parallel),)
 ITEST_PARALLELISM = $(parallel)
 endif
 
+# Windows needs to append a .exe suffix to all executable files, otherwise it
+# won't run them.
+ifneq ($(windows),)
+EXEC_SUFFIX = .exe
+endif
+
 # If specific package is being unit tested, construct the full name of the
 # subpackage.
 ifneq ($(pkg),)