* fix: convert error output to be consistent cross-platform
The error output produced by `shell.error()` or `result.stderr` should
not be inconsistent between platforms. This ensures that path separators
are always printed by ShellJS as `/` instead of as `\` on Windows. This
should allow scripts using ShellJS to be more consistent cross-platform.
We were not previously relying on error output to always be consistent--
only checking its truthiness. Since this was not part of our tested API,
it should be reasonable to change this and release in a patch.
Fixes#681
* Fix broken pushd test case
* Fix TODO in a test case