tests: on_target: small fix.

Another small fix.

Signed-off-by: Giacomo Dematteis <giacomo.dematteis@nordicsemi.no>

Author: DematteisGiacomo

.github/workflows/target-test.yml

@@ -46,7 +46,7 @@ jobs:
         include:
           - device: cia-trd-thingy91x
           - device: ppk_thingy91x
-            if: ${{ inputs.pytest_marker != 'not slow' }}
+            if: ${{ inputs.pytest_marker == '' || inputs.pytest_marker == 'no_markers_flag' || inputs.pytest_marker == 'slow' }}
 
     # Self-hosted runner is labeled according to the device it is linked with
     runs-on: ${{ matrix.device }}