[libc++] Consider everything inside %T to be a dependency of each test

Instead of passing file dependencies individually, assume that the
whole content of the unique test directory is a dependency. This
simplifies the test harness significantly, by making %T the directory
that contains everything required to run a test. This also removes the
need for the %{file_dependencies} substitution, which is removed by this
patch.

Furthermore, this patch also changes the harness to execute tests locally
inside %T, so as to avoid creating a separate directory for no purpose.