selftests: mptcp: userspace_pm: fixed subtest names

It is important to have fixed (sub)test names in TAP, because these
names are used to identify them. If they are not fixed, tracking cannot
be done.

Some subtests from the userspace_pm selftest were using random numbers
in their names: the client and server address IDs from $RANDOM, and the
client port number randomly picked by the kernel when creating the
connection. These values have been replaced by 'client' and 'server'
words: that's even more helpful than showing random numbers. Note that
the addresses IDs are incremented and decremented in the test: +1 or -1
are then displayed in these cases.

Not to loose info that can be useful for debugging in case of issues,
these random numbers are now displayed at the beginning of the test.

Fixes: f589234e ("selftests: mptcp: userspace_pm: format subtests results in TAP")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240614-upstream-net-20240614-selftests-mptcp-uspace-pm-fixed-test-names-v1-1-460ad3edb429@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>