Let $\Omega \subset \mathbb{R}^n$ ($n \geq 1$) be a bounded open domain and $f \in L^\infty(\Omega)$ possibly changes the sign. Assume that the set $$ \Omega^+ := \{x \in \Omega: f(x) > 0 \} $$ has positive measure. Is it true that there exists non-negative $\varphi \in C_0^1(\Omega)$, such that $$ \int_\Omega f \, \varphi \, dx > 0 ~? $$ Obviously, if $\Omega^+$ has nonempty interior (after redefinition on a set of measure $0$), then the answer is Yes. But I'm afraid of the case $\Omega^+$ is a "bad" set, in the sense that its interior is empty (like fat Cantor set).
Thanks.