tests/build: Iterate drivers_tests as a dict when skipping driver tests

bf91b0ed ("tests/build: Allow to define per-test parameters via a dict")
turned drivers_tests into a dict and updated the loop that defines the real
tests, but the fallback that only adds the skipping ones still asks for a
single iteration variable. Configuring with -Dintrospection=false stops right
there:

  tests/meson.build:295:25: ERROR: Foreach expects exactly 2 variables for
  iterating over objects of type dict

Hit it with meson 1.11 while building a single driver without introspection.

tests/meson.build

@@ -293,7 +293,7 @@ else
         args: ['-c', 'exit 77']
     )
 
-    foreach driver_test: drivers_tests
+    foreach driver_test, _args : drivers_tests
         test(driver_test,
             sh,
             args: ['-c', 'exit 77']