tests: move DEFAULT_STRLEN macro to tests.h

tests: move DEFAULT_STRLEN macro to tests.h

* tests/tests.h [!DEFAULT_STRLEN] (DEFAULT_STRLEN): New macro.
* tests/mincore.c (DEFAULT_STRLEN): Remove.
* tests/mmsg_name.c: Likewise.
* tests/msg_control.c: Likewise.
* tests/netlink_protocol.c: Likewise.
* tests/nlattr.c: Likewise.
* tests/nlattr_inet_diag_msg.c: Likewise.
* tests/nlattr_netlink_diag_msg.c: Likewise.
* tests/nlattr_unix_diag_msg.c: Likewise.
* tests/printstr.c: Likewise.
* tests/xattr.c: Likewise.
* tests/execve.c: Likewise.
(main): Initialize str_a and str_b arrays at run time.
* tests/execveat.c: Likewise.

Suggested-by: JingPiao Chen <chenjingpiao@gmail.com>