[libc++][test] try to directly create socket file in /tmp when filepath is too long (#77058)

If TMP is set to a folder which path is too long, the current libcxx
test helper function `create_socket()` will fail because of the test
temp folder `test_root`'s path is too long to be used in socket
creation.
In such case, this patch will try to create the socket file directly in
`/tmp` folder.

This patch also add an assertion for `bind()`.