Use cowlib instead of rebar for core-deps-build-erl

Ran into weird Rebar issues on Windows, I will probably run into
those issues again later on, but this test isn't about that.

test/core_deps.mk

@@ -81,23 +81,22 @@ core-deps-build-erl: build clean-core-deps-build-erl
 	$t cp ../erlang.mk $(APP)/
 	$t $(MAKE) -C $(APP) -f erlang.mk bootstrap-lib $v
 
-	$i "Add rebar to the list of build dependencies"
+	$i "Add cowlib to the list of build dependencies"
 	$t sed -i.bak '2i\
-BUILD_DEPS = rebar\
-dep_rebar = git https://github.com/rebar/rebar master\
+BUILD_DEPS = cowlib\
 ' $(APP)/Makefile
 
 	$i "Build the application"
 	$t $(MAKE) -C $(APP) $v
 
 	$i "Check that all dependencies were fetched"
-	$t test -d $(APP)/deps/rebar
+	$t test -d $(APP)/deps/cowlib
 
 	$i "Check that the application was compiled correctly"
 	$t $(ERL) -pa $(APP)/ebin/ $(APP)/deps/*/ebin/ -eval " \
-		[ok = application:load(App) || App <- [$(APP), rebar]], \
+		[ok = application:load(App) || App <- [$(APP), cowlib]], \
 		{ok, Deps} = application:get_key($(APP), applications), \
-		false = lists:member(rebar, Deps), \
+		false = lists:member(cowlib, Deps), \
 		halt()"
 
 core-deps-pkg: build clean-core-deps-pkg