fable: Improve Conf documentation and fix pointer related bugs

During writing tests for the documentation, some bugs related
to the use of JSON pointer in error messages were found and are
corrected in this commit.

Author: cassava

fable/CMakeLists.txt

@@ -63,6 +63,7 @@ else()
 endif()
 
 # Testing ------------------------------------------------------------
+set(CMAKE_CTEST_ARGUMENTS "--output-on-failure")
 include(CTest)
 if(BUILD_TESTING)
     find_package(GTest REQUIRED)
@@ -74,6 +75,7 @@ if(BUILD_TESTING)
         # find src -type f -name "*_test.cpp"
         src/fable/version_test.cpp
         src/fable/environment_test.cpp
+        src/fable/conf_test.cpp
         src/fable/schema/array_test.cpp
         src/fable/schema/const_test.cpp
         src/fable/schema/custom_test.cpp