Question

Why potential energy for a test charge with negative charge on it is negative and it’s vice versa?

Answer 1

The electric potential $V_p$ at a point P due to a single point charge +Q is
$V_p=\frac{Q}{4\pi {ϵ}_{0}r}$
Where r is the distance of point P from the charge +Q.
$V_P=9\times {10}^9\frac{Q}{r}$... in free space/air
$V_P=9\times {10}^9\frac{Q}{{ϵ}_{r}r}$ ... in a medium

If Q is positive, then potential at P is positive. On the other hand, if Q is negative, then potential at P is negative. It is reminded that electric potential is a scalar quantity.
A positive charge loses electric potential energy as it moves from high potential to low potential and vice-versa. On the other hand, a negative charge gains electric potential energy as it moves from high to low potential and vice-versa.