perfect_gas.test - Fixes to decrease false failure rates.

[dreamer2368]

perfect_gas.test sets a random state and gradient to test PerfectMixture class methods.
This state and gradient are occasionally set to outliers and cause a false failure for ComputePressureDerivative test, mainly for two reasons:

• When the randomly chosen pressure gradient is too small compared to pressure magnitude, its relative error goes beyond a threshold by losing digits in arithmetic operations.
• When the randomly chosen gradient is large, the initial step size for finite-difference approximation is too large to perturb the state unto an unnatural state (mainly negative densities).

These are false failures, not due to actual malfunctioning of class methods. To decrease false-failure rates, following fixes are made:

• If the relative error is larger than threshold (1e-13), it also checks the absolute error (<1e-8). For a reference, minimum pressure gradient magnitude is around 1e2.
• The initial step size for finite-difference approximation is chosen dynamically based on the state and gradient, so that the perturbed state wouldn't violate necessary conditions.

After this fix, no false failure was found from 100 tests. (used to be ~10).