socket tests: Clean up the MSG_TRUNC regression tests a bit

• Fix style.
• Move test case-specific code out of the shared function and into the individual test cases.
• Remove unneeded setting of SO_REUSEPORT.
• Avoid unnecessary copying.
• Use ATF_REQUIRE* instead of ATF_CHECK*. The former cause test execution to stop after a failed assertion, which is what we want.
• Add a test case for AF_LOCAL/SOCK_SEQPACKET sockets.

MFC after: 1 week