Test to copy directory with files with same name prefix

[john-jam]

One test case in gcsfs had a weird behavior where copying a folder also copied the files that has the same name prefix as the folder name. For example:

# Files
# -- subdir.txt
# -- subdir
# ---- subfile.txt

fs.cp("subdir", "target")

# Expected target folder content: ["subfile.txt"]

# Content with gcsfs: ["subdir.txt", "subdir/subfile.txt"]

This PR adds tests to cover this use case.

@ianthomas23 @martindurant I created a PR here for now since I need the updates from this other PR for it to work. I'll create a PR with fsspec master branch as the target as soon as the other one is merged.