Fixes #3059.

cabal's handling of non-existent sources depends on the behavior of the
directory package. 'canonicalizePath' can fail on non-existent paths before
directory-1.2.3.0. This commit updates the test 'fail_on_nonexistent_source' to
allow 'cabal sandbox delete-source' to fail or succeed.  It also changes
'fail_removing_source_thats_not_registered' so that it only tests existing
sources.

(cherry picked from commit 4a06b1d4)