Fix small issue with c-src tests

Main test Makefile expects "c_src" but for convenience I also
allow "c-src" and use that one as the prefix for the tests.

test/plugin_c_src.mk

@@ -7,11 +7,13 @@ C_SRC_CLEAN_TARGETS = $(addprefix clean-,$(C_SRC_TARGETS))
 .PHONY: c-src $(C_SRC_TARGETS) clean-c-src $(C_SRC_CLEAN_TARGETS)
 
 clean-c-src: $(C_SRC_CLEAN_TARGETS)
+clean-c_src: clean-c-src
 
 $(C_SRC_CLEAN_TARGETS):
 	$t rm -rf $(APP_TO_CLEAN)/
 
 c-src: $(C_SRC_TARGETS)
+c_src: c-src
 
 c-src-nif: build clean-c-src-nif