[nrf fromtree] tests: drivers: comparator: gpio_loopback: Update SNIPPET_ROOT

When SNIPPET is added to the build, it is added to all images. But not
all images have the test directory added to their SNIPPET_ROOT, so we
also update the SNIPPET_ROOT.

This allows, for instance, the uicr image of nrf54H to find this
snippet.

Ref: NCSDK-NONE

Signed-off-by: Sebastian Bøe <[email protected]>
(cherry picked from commit 5f9d20c)

Author: SebastianBoe

tests/drivers/comparator/gpio_loopback/testcase.yaml

@@ -18,6 +18,7 @@ tests:
       - frdm_ke15z
   drivers.comparator.gpio_loopback.nrf_comp:
     extra_args:
+      - SNIPPET_ROOT="."
       - SNIPPET="gpio_loopback_nrf_comp"
     platform_allow:
       - nrf5340dk/nrf5340/cpuapp
@@ -27,6 +28,7 @@ tests:
       - ophelia4ev/nrf54l15/cpuapp
   drivers.comparator.gpio_loopback.nrf_lpcomp:
     extra_args:
+      - SNIPPET_ROOT="."
       - SNIPPET="gpio_loopback_nrf_lpcomp"
     platform_allow:
       - nrf5340dk/nrf5340/cpuapp