tests: Gracefully handle EADDRINUSE string from musl.

tests: Gracefully handle EADDRINUSE string from musl.

Different libc implementations may have different string representations
for the same errno. Service shutdown routines verify that no
*unexpected* warnings are seen in service logs. This makes the test case
fail because a slightly different warning is observed.

This patch makes the test case gracefully handle the musl libc
representation.

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>