checkpoints: introduce TST_CHECKPOINT_CREATE

Rename tst_checkpoint_init to tst_checkpoint_create.
Add tst_checkpoint_init, which only initializes checkpoint struct.
Update existing testcases to use TST_CHECKPOINT_CREATE.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>
checkpoints: fix usage in children started via exec

creat07 and setpgid03 are currently failing in following way:
  CHILD: Failed to open fifo '': No such file or directory at creat07_child.c:31
  creat07     1  TBROK  :  tst_checkpoint.c:126: Checkpoint timeouted after 5000 msecs at creat07.c:78
  creat07     2  TBROK  :  tst_checkpoint.c:126: Remaining cases broken

These testcases start children via exec, so only initialize
checkpoint struct, FIFO has been already created by parent.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Acked-by: Cyril Hrubis <chrubis@suse.cz>