[MQTT] Simplifies MQTT CMakeLists file

- Uses mocks from tools instead of creating them.
- Move host based definition to test code.

tools/mocks/http_parser/CMakeLists.txt

@@ -0,0 +1,8 @@
+# NOTE: This kind of mocking currently works on Linux targets only.
+#       On Espressif chips, too many dependencies are missing at the moment.
+message(STATUS "building HTTP PARSER MOCKS")
+
+idf_component_get_property(original_http_parser_dir http_parser COMPONENT_OVERRIDEN_DIR)
+
+idf_component_mock(INCLUDE_DIRS "${original_http_parser_dir}"
+    MOCK_HEADER_FILES ${original_http_parser_dir}/http_parser.h)

tools/mocks/http_parser/mock/mock_config.yaml

@@ -0,0 +1,9 @@
+:cmock:
+  :plugins:
+    - expect
+    - expect_any_args
+    - return_thru_ptr
+    - array
+    - ignore
+    - ignore_arg
+    - callback