Merge ENABLE_THREADS and ENABLE_STD_THREADS markers
Both of these markers are used in the test suit for annotating when a test needs multi threaded support. Previously they had slightly different meaning but they converged to the point where they are used interchangeably. This CL removes the ENABLE_STD_THREADS one to simplify the test suite and avoid some confusion.
Note: This CL is a preparation to get test running on android compiled with clang