selftests: mptcp: diag: unique 'cestab' subtest names

It is important to have a unique (sub)test name in TAP, because some CI
environments drop tests with duplicated name.

Some 'cestab' subtests from the diag selftest had the same names, e.g.:

    ....chk 0 cestab

Now the previous value is taken, to have different names, e.g.:

    ....chk 2->0 cestab after flush

While at it, the 'after flush' info is added, similar to what is done
with the 'in use' subtests. Also inspired by these 'in use' subtests,
'many' is displayed instead of a large number:

    many msk socket present                           [  ok  ]
    ....chk many msk in use                           [  ok  ]
    ....chk many cestab                               [  ok  ]
    ....chk many->0 msk in use after flush            [  ok  ]
    ....chk many->0 cestab after flush                [  ok  ]

Fixes: 81ab7728

 ("selftests: mptcp: diag: check CURRESTAB counters")
Cc: stable@vger.kernel.org
Reviewed-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>