[OMPT] Add tool_not_available testcase

Add a testcase that checks wheter the runtime can handle an ompt_start_tool
method that returns NULL indicating that no tool shall be loaded.

All tool_available testcases need a separate folder to avoid file conflicts for
the generated tools.

Differential Revision: https://reviews.llvm.org/D41904

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@324587 91177308-0d34-0410-b5e6-96231b3b80d8