core: add CMake test called "notests" when tests are not compiled

Now the command `make test` executes properly and returns 0 when test are not
compiled.

CMakeLists.txt

@@ -241,6 +241,9 @@ if(ENABLE_TESTS)
   else()
     message(SEND_ERROR "CppUTest not found")
   endif()
+else()
+  enable_testing()
+  add_test(NAME notests COMMAND true)
 endif()
 
 configure_file(config.h.cmake config.h @ONLY)

ChangeLog.adoc

@@ -65,6 +65,7 @@ Tests::
 
 Build::
 
+  * core: add CMake test called "notests" when tests are not compiled
   * core: rename scripts: build-debian.sh to build_debian.sh, build-test.sh to build_test.sh, git-version.sh to set_git_version.sh
   * debian: change dependency guile-2.2-dev to guile-3.0-dev
   * python: remove support of Python 2.x