tests: net: reuseaddr_reuseport: Fix zassert statements

One should not call a function directly from within zassert()
as it can cause the value of errno to be lost as seen by this
failed test output

Assertion failed at tests/net/socket/reuseaddr_reuseport/src/main.c:151:
  test_bind_success: (zsock_bind(sock, addr, addrlen) not equal to 0)
  bind() failed with error 0
FAIL - test_ipv4_udp_bad_both_not_set in 0.000 seconds

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>