tests/btrfs.c: fix build on systems without <linux/btrfs.h> or <linux/fiemap.h>
tests/btrfs.c: fix build on systems without <linux/btrfs.h> or <linux/fiemap.h>

* tests/btrfs.c: Include "tests.h" first.
Skip the test if [!HAVE_LINUX_BTRFS_H].  Skip parts of the test
that use struct fiemap if [!HAVE_LINUX_FIEMAP_H].

cuviper/strace,cuviper/strace,Saruta/strace,cuviper/strace,Saruta/strace,cuviper/strace,Saruta/strace,cuviper/strace,Saruta/strace,Saruta/strace