Add accelerometer self-test function to LIS2DW12 driver

[Ctru14]

Add accelerometer self-test function to LIS2DW12 driver.

Run accelerometer self-test with positive and negative sign based on STMicro specifications and example:
https://github.com/STMicroelectronics/STMems_Standard_C_drivers/blob/master/lis2dw12_STdC/examples/lis2dw12_self_test.c

Output with extra prints added:

Unbiased average: X=0.5859375, Y=-5.9570313, Z=1017.77344 (191 Attempts / 5 Samples)
Positive average: X=275.48828, Y=220.11719, Z=1628.9063 (177 Attempts / 5 Samples)
Negative average: X=-214.45313, Y=-262.3047, Z=616.1133 (186 Attempts / 5 Samples)
Accel self-test diffs in mGs (Expected between 70-1500):
Positive X=274.90234, Y=226.07422, Z=611.1328
Negative X=215.03906, Y=256.34766, Z=401.66016
Accel self-test passed!