tests: Allow run_make_test() to avoid passing -f

tests: Allow run_make_test() to avoid passing -f

We already use undef makefile strings to mean "re-use the previous
makefile", so if the string is empty we'll assume it means "don't
use a makefile at all" (don't add -f).

* tests/run_make_tests.pl (run_make_test): If the makefile string
is empty, don't create a makefile or add -f.
* tests/scripts/features/include: Change empty makefile to "\n".
* tests/scripts/misc/close_stdout: Ditto.
* tests/scripts/options/dash-r: Ditto.