integ-cli: Fix path issues in docker cp tests

Some of the `docker cp` tests were using `path/filepath` to
craft unix paths. This wouldn't work on Windows since filepath
is platform-dependent.

Moved code to `path` as much as possible and hacked away some
`path/filepath` functionality that doesn't exist in `path` pkg.

This fixes the following test cases:
- `TestCpGarbagePath`
- `TestCpRelativePath`
- `TestCpAbsoluteSymlink`
- `TestCpSymlinkComponent`

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>