Exec tests no longer fail if there isn't a sandbox

The tests which delete the sandbox, now check for its existence before
deleting it. This prevents erroneous test failures when the tests are first
run or any time that the sandbox has been deleted.
cabal-install tests use locally built cabal

When running the cabal-install tests, we want to run against the locally built
cabal executable, not any other cabal executable which may be installed.

If we are 1) following the advice of building cabal with `cabal build` (as
opposed to installing it into a sandbox with `cabal install`) and 2) not using
a non-default build directory, the locally built cabal will be located at
`dist/build/cabal/cabal`.

By adding the directory `dist/build/cabal` to the program search path we will
pick up any cabal executable there in preference to any other on the $PATH.

For most users, I expect that this will remove any surprises with an
unexpected cabal executable being selected for running the tests. It will be
sufficient to allow the tests to run on Travis.

For users who have been installing cabal into a sandbox, this change could
result in an older cabal executable being selected. It's not clear to me how
to solve this issue or whether anyone will experience this.
Enable cabal-install tests on travis