feat(heap): Update heap tests on external memory

Create a specific group "psram" to only run meaningful
tests on the sdkconfig.ci.psram and sdkconfig.ci.psram_all_ext
configuration.

Add test testing that external memory is prioritized when using
MALLOC_CAP_DEFAULT.

components/heap/test_apps/heap_tests/pytest_heap.py

@@ -66,7 +66,7 @@ def test_heap_in_flash(dut: Dut) -> None:
     ]
 )
 def test_heap(dut: Dut) -> None:
-    dut.run_all_single_board_cases()
+    dut.run_all_single_board_cases(group='psram')
 
 
 @pytest.mark.generic