RSL10-SENSE-GEVK Read analog signals on accessible pins
As per RSL10 datasheet:


  An Analog to Digital converter (ADC), accessed by the
  Arm Cortex−M3 processor. The ADC can read 4
  external values (DIO[0]−DIO[3]), AOUT, VDDC,
  VBAT/2 and the ADC offset value.


While the wording is a little bogus, you should be able to use AOUT as input to the SoC's ADC, which will read the voltage on AOUT. However I am not sure whether this means that you could connect an external voltage source to AOUT or if this feature is only supposed to read the SoC's VDDA, which is the power domain for AOUT as per datasheet.

Adding to the confusion: AOUT is designated as OUTPUT only in the datasheet.


  If that isn't possible, can the GIO_SPARE read an analog input and use AOUT as a digital output?


I did not find GIO_SPARE (nor GPIO_SPARE) in the datasheet, but yes, AOUT can be used as digital output.

Anyway, I'd recommend using the designated I/O pins instead of testpoints:


  RSL10 includes 16 DIO pads (Digital Input/Output) that all
  can be assigned to any of the interfaces above, or used as
  general purpose DIOs.


If you can't do that and must use AOUT, you should configure it as input to the ADC, and then try to measure if theres any voltage on AOUT by using an external voltmeter. If there is, you either configured it wrong or it's simply not intended to be used as analog input.