The attr typically located on the stack is of an opaque pthread_attr_t
type, which may be a pointer that gets initialized by
pthread_attr_init(). Explicitly clean up the attr with
pthread_attr_destroy() to avoid a leak on such platforms to avoid
unexpected test failures with lsan enabled.
This primarily affects FreeBSD; NetBSD, musl, and glibc will seemingly
all use a full-sized pthread_attr_t.
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