[libc++] Run tests in a directory related to %t instead of /tmp

Instead of creating a temporary directory inside /tmp and running the
tests there, use a directory name based on LIT's %t substitution. This
has the benefit of not hitting /tmp so much (which is slow on some
filesystems). It also has the benefit that `ninja -C build clean` will
automatically remove the artifacts even if a test somehow failed to
remove its temporary directory (I've seen this happen when CTRL-C is
received).