cli/command/container: TestSplitCpArg: cleaner skip

[thaJeztah]

• relates to https://github.com/docker/cli/pull/5224#discussion_r1665426727

Trying to make the logic slightly clearer, and adding a custom message for the skip,

Before this:

=== RUN   TestSplitCpArg/absolute_path_with_drive
    cp_test.go:184: tc.os == "windows" && runtime.GOOS != "windows" || tc.os == "linux" && runtime.GOOS == "windows"

After this:

=== RUN   TestSplitCpArg/absolute_path_with_drive
    cp_test.go:185: skipping windows test on darwin