Return early by checking _FILE_INCLUDED property as soon as possible
Fix typo in invocation of superbuild_message within superbuild_include_dependencies
Rename tests and add convenience macro to easily add more test cases

Added 'add_project_test' macro allowing to test if a project configure
and build as expected.
Add support for SUPERBUILD_VAR argument.

If not specified, it defaults to ${SUPERBUILD_TOPLEVEL_PROJECT}_SUPERBUILD
Add support for multiple call to 'superbuild_include_dependencies;

This commit unset global variables at the end of
superbuild_include_dependencies to make sure their value won't
have side effect with following call to the macro.
Ensure DEPENDS_VAR is properly updated and add associated test case.
Add all test helper functions/macros to ArtichokeTestUtility.cmake module
Update MulitCall testcase to consider a dependency tree with optional deps
Rename macro to match ExternalProject naming convention

See #2