[OMPT] Add tool_available_search testcase

Tests the search for tools as defined in the spec. The OMP_TOOL_LIBRARIES
environment variable contains paths to the following files(in that order)

-to a nonexisting file
-to a shared library that does not have a ompt_start_tool function
-to a shared library that has an ompt_start_tool implementation returning NULL
-to a shared library that has an ompt_start_tool implementation returning a
    pointer to a valid instance of ompt_start_tool_result_t

The expected result is that the last tool gets active and can print in the
thread-begin callback.

Differential Revision: https://reviews.llvm.org/D42166

llvm-svn: 324588