Remove unused test files

Following commit f534e18, the External_* files used by the MultiCall test
are automatically generated. This commit remove the now unused file.
Fix CMakeLists in MultiCall test to work as expected in the inner build case
Properly set or restore the project variable. Fixes #4

In case no PROJECT_VAR argument is passed, the variable "proj" is considered
and set to <projectname>.

For the second pass, the value of "${_sb_PROJECT_VAR}" is used instead
of SUPERBUILD_TOPLEVEL_PROJECT.