chore: rename targets and add ENABLE_TESTING option

Rename library targets from celrs_* to cel* (celcrsf, celserial,
cellogger, cellog) and add cel:: namespace aliases. Add cel::cel
umbrella target that links all core libraries.

Add ENABLE_TESTING option (default ON) to gate Unity/CMock fetch
and test targets for downstream consumers.

CMakeLists.txt

@@ -5,6 +5,9 @@ project(celrs VERSION 0.1.0)
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/deps")
 
+# Options
+option(ENABLE_TESTING "Build unit tests" ON)
+
 # Platform flags
 include(Platform)
 include(Flags)
@@ -18,8 +21,10 @@ add_subdirectory(celrs)
 add_subdirectory(tools)
 
 # Testing
-enable_testing()
-add_subdirectory(tests)
+if (ENABLE_TESTING)
+    enable_testing()
+    add_subdirectory(tests)
+endif()
 
 # IDE configuration
 include(IDE)