[unittest] Skip the socket tests if we $TMPDIR is too long.

Adrian added a sanity check to the socket tests to ensure the $TMPDIR is
not too long for a socket. While this is great for diagnosing the
problem it doesn't really solve the problem for environment where you
have no control over that variable such as in CI. I propose to just skip
the test in that case similar to what we do for tests that rely on
targets that are not currently build, etc.

Differential revision: https://reviews.llvm.org/D67972

llvm-svn: 372774