Fix skriptTest and similar tasks not working on Windows

[Nicofisi]

Description

This PR fixes the skriptTest Gradle task not working on Windows, and the two other test tasks. It does so by changing the classpath file separator from : to ${File.pathSeparator}, which stands for : on Linux and Mac, and ; on Windows.

Relevant JavaDoc:

    /**
     * The system-dependent path-separator character.  This field is
     * initialized to contain the first character of the value of the system
     * property <code>path.separator</code>.  This character is used to
     * separate filenames in a sequence of files given as a <em>path list</em>.
     * On UNIX systems, this character is <code>':'</code>; on Microsoft Windows systems it
     * is <code>';'</code>.
     *
     * @see     java.lang.System#getProperty(java.lang.String)
     */
    public static final char pathSeparatorChar = fs.getPathSeparator();

    /**
     * The system-dependent path-separator character, represented as a string
     * for convenience.  This string contains a single character, namely
     * <code>{@link #pathSeparatorChar}</code>.
     */
    public static final String pathSeparator = "" + pathSeparatorChar;

Thanks for helping to test on Mac @ShaneBeee ;) 🌵

---

Target Minecraft Versions: any
Requirements: none
Related Issues: #2797

[ShaneBeee]

Thanks for helping to test on Mac @ShaneBeee ;) 🌵

You're welcome!

As Nico stated, I tested this on macOS and it works great 👍
Appears to build properly in the actions.

Looks good to me :)