Using Button from WordPress components does not work when used for frontend

This isn’t how the WordPress components are supposed to be used. The components from @wordpress/components are meant for use in the admin, typically for creating a UI in the block editor. See how the package is described in the documentation (emphasis mine):

This package includes a library of generic WordPress components to be
used for creating common UI elements shared between screens and
features of the WordPress dashboard.

One of the uses for this is to create elements that use the styles of the WordPress admin. If you expected this component to use your theme’s button styles on the front-end, then you’re mistaken.

If you want a display a button in your block that matches your theme’s styles (assuming a modern block-based theme) then you should just use the normal HTML, but with the wp-element-button class:

import { registerBlockType } from '@wordpress/blocks';

registerBlockType('mynamespace/buttontest', {
    title: 'mynamespace/buttontest',
    edit: () => {
        return (
            <button className="wp-element-button">Test</button>
        );
    },
    save: () => {
        return (
            <button className="wp-element-button">Test2</button>
        );
    }
})