tests: get rid of test-helper.[ch] completely

I don't think there's any particular reason to keep those functions in a separate
file.

src/test/test-engine.c

@@ -7,7 +7,6 @@
 #include "manager.h"
 #include "rm-rf.h"
 #include "strv.h"
-#include "test-helper.h"
 #include "tests.h"
 #include "service.h"