tests/kernel/mem_pool: Split out tests for legacy mem_pool config

The k_heap backend is now the default for mem_pool, so duplicate these
tests across that config so we continue to have coverage for the older
code.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

tests/kernel/mem_pool/mem_pool/testcase.yaml

@@ -2,3 +2,8 @@ tests:
   kernel.memory_pool:
     min_ram: 32
     tags: kernel mem_pool
+  kernel.memory_pool.legacy:
+    min_ram: 32
+    tags: kernel mem_pool
+    extra_configs:
+      - CONFIG_MEM_POOL_HEAP_BACKEND=n

tests/kernel/mem_pool/mem_pool_api/testcase.yaml

@@ -1,3 +1,7 @@
 tests:
   kernel.memory_pool.api:
     tags: kernel mem_pool
+  kernel.memory_pool.api.legacy:
+    tags: kernel mem_pool
+    extra_configs:
+      - CONFIG_MEM_POOL_HEAP_BACKEND=n

tests/kernel/mem_pool/mem_pool_concept/testcase.yaml

@@ -1,3 +1,7 @@
 tests:
   kernel.memory_pool.concept:
     tags: kernel mem_pool
+  kernel.memory_pool.concept.legacy:
+    tags: kernel mem_pool
+    extra_configs:
+      - CONFIG_MEM_POOL_HEAP_BACKEND=n

tests/kernel/mem_pool/mem_pool_threadsafe/testcase.yaml

@@ -1,3 +1,7 @@
 tests:
   kernel.memory_pool.threadsafe:
     tags: kernel mem_pool
+  kernel.memory_pool.threadsafe.legacy:
+    tags: kernel mem_pool
+    extra_configs:
+      - CONFIG_MEM_POOL_HEAP_BACKEND=n