Add unit tests

[danieldietrich]

There are already many edge cases (and their combinations) to be tested

• eager and lazy web-imports
• cascaded web-components and web-imports
• cyclic components and imports
• absolute and relative imports (also bouncing between different servers)
• Content-Security-Policy (CSP) regarding eval
• creating user defined elements (without extending tags)
• extending native elements
• order of lifecycle method calls, especially on async initialization and when manually creating an element using document.createElement('my-elem') + adding attributes and properties before connecting the element to the live DOM
• use no mode, mode open and mode closed
• etc.