selftests: mptcp: diag: unique 'in use' subtest names

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

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

    chk 0 msk in use after flush

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

    chk 2->0 msk in use after flush

While at it, avoid repeating the full message, declare it once in the
helper.

Fixes: ce990257

 ("selftests: mptcp: diag: format subtests results in TAP")
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>