test/fuzz: add a function for generating fuzz test targets

Commit 2be74a65 ("test/cmake: add a function for generating unit
test targets") added a function for generating unit test targets in
CMake. This function makes code simpler and less error-prone.

Proposed patch adds a similar function for generating fuzzing test
targets in CMake.

NO_CHANGELOG=build infrastructure updated
NO_DOC=build infrastructure updated
NO_TEST=build infrastructure updated