Add unit test for custom mobility model

#DESC tests selection of a custom mobility model

[Allpix]

detectors_file = "detector.conf"

number_of_events = 1

random_seed = 0

multithreading = true

workers = 3

[DepositionPointCharge]

model = "fixed"

source_type = "point"

position = 0,0,0

[GenericPropagation]

temperature = 293K

charge_per_step = 100

log_level = DEBUG

propagate_electrons = true

propagate_holes = true

# Replicating the Jacoboni-Canali mobility model at T = 293K

mobility_model = "custom"

mobility_function_electrons = "[0]/[1]/pow(1.0+pow(x/[1],[2]),1.0/[2])"

mobility_parameters_electrons = 1.0927393e7cm/s, 6729.24V/cm, 1.0916

mobility_function_holes = "[0]/[1]/pow(1.0+pow(x/[1],[2]),1.0/[2])"

mobility_parameters_holes = 8.447804e6cm/s, 17288.57V/cm, 1.2081

#PASS (DEBUG) [I:GenericPropagation:mydetector] Selected mobility model "custom"

#LABEL coverage

#FAIL ERROR

#FAIL FATAL