Do not check dir perms when .ghci doesn't exist, otherwise GHCi will
print some confusing and useless warnings in some cases (e.g. in travis).
This will fix test T8333 and T10408A in travis.

T10408A will be a test case to cover this. And T8333 is changed to be
not affected by this.

Test Plan:
  chmod o+w ~/.ghc
  make TESTS="T8333 T10408A T10408B"
  chmod o-w ~/.ghc

Reviewers: austin, nomeata

Differential Revision: https://phabricator.haskell.org/D890