Cleanup non-standard tests as reported by STL@microsoft.com. NFC.

This patch addresses the following issues in the test suite:

1. Move "std::bad_array_length" test from std/ to libcxx/ test directory
   since the feature is not a part of the standard.

2. Rename "futures.tas" test directory to "futures.task" since that is the
   correct stable name.

3. Move tests for "packaged_task<T>::result_type" from std/ to libcxx/
   test directory since the typedef is a libc++ extension.

llvm-svn: 271430