selftests: mptcp: join: disable get and dump addr checks

These new checks have been recently queued to v6.6 [1] with the backport
of commit 38f027fc ("selftests: mptcp: dump userspace addrs list"),
and commit 4cc5cc7c ("selftests: mptcp: userspace pm get addr
tests").

On v6.6, these checks will simply print 'skip', because the associated
features are not available in this version. That's fine, except that the
MPTCP CI sets the SELFTESTS_MPTCP_LIB_EXPECT_ALL_FEATURES=1 env var,
which will force these subtests to fail when using the selftests from
v6.6 on a v6.6 kernel, because the feature is not available.

To ease the backports (and possible future ones), I suggest to keep the
recent backports, but skip calling mptcp_lib_kallsyms_has() not to have
the CIs setting this env var complaining about the associated features
not being available.

Link: https://git.kernel.org/pub/scm/linux/kernel/git/stable/stable-queue.git/commit/?id=bd2122541bd8

 [1]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>