selftests/landlock: Add network tests

Add 82 test suites to check edge cases related to bind() and connect()
actions. They are defined with 6 fixtures and their variants:

The "protocol" fixture is extended with 12 variants defined as a matrix
of: sandboxed/not-sandboxed, IPv4/IPv6/unix network domain, and
stream/datagram socket. 4 related tests suites are defined:
* bind: Tests bind action.
* connect: Tests connect action.
* bind_unspec: Tests bind action with the AF_UNSPEC socket family.
* connect_unspec: Tests connect action with the AF_UNSPEC socket family.

The "ipv4" fixture is extended with 4 variants defined as a matrix
of: sandboxed/not-sandboxed, and stream/datagram socket. 1 related test
suite is defined:
* from_unix_to_inet: Tests to make sure unix sockets' actions are not
  restricted by Landlock rules applied to TCP ones.

The "tcp_layers" fixture is extended with 8 variants defined as a matrix
of: IPv4/IPv6 network domain, and different number of landlock rule
layers. 2 related tests suites are defined:
* ruleset_overlap: Tests nested layers with less constraints.
* ruleset_expand: Tests nested layers with more constraints.

In the "mini" fixture 4 tests suites are defined:
* network_access_rights: Tests handling of known access rights.
* unknown_access_rights: Tests handling of unknown access rights.
* inval: Tests unhandled allowed access and zero access value.
* tcp_port_overflow: Tests with port values greater than 65535.

The "ipv4_tcp" fixture supports IPv4 network domain with stream socket.
2 tests suites are defined:
* port_endianness: Tests with big/little endian port formats.
* with_fs: Tests a ruleset with both filesystem and network
  restrictions.

The "port_specific" fixture is extended with 4 variants defined
as a matrix of: sandboxed/not-sandboxed, IPv4/IPv6 network domain,
and stream socket. 2 related tests suites are defined:
* bind_connect_zero: Tests with port 0.
* bind_connect_1023: Tests with port 1023.

Test coverage for security/landlock is 92.4% of 710 lines according to
gcc/gcov-13.

Signed-off-by: Konstantin Meskhidze <konstantin.meskhidze@huawei.com>
Link: https://lore.kernel.org/r/20231026014751.414649-11-konstantin.meskhidze@huawei.com


[mic: Extend commit message, update test coverage, clean up capability
use, fix useless TEST_F_FORK, and improve ipv4_tcp.with_fs]
Co-developed-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Mickaël Salaün <mic@digikod.net>