Use `set -gx` instead of `setenv` for fish shell.
Fix test "activate if the first virtualenv is a virtualenv": it was asserting
the output was of bash form, but not specifying that in the test — it now does.
Fixes https://github.com/pyenv/pyenv-virtualenv/issues/216.
This means that pyenv-virtualenv will not allow manual deactivation of
virtualenv if pyenv-virtualenv-init is enabled. This must be acceptable
since the activation of virtualenv is just setting of environment
variables in current implementation.
