Ideas for additional coverage for base64

[Ms2ger]

Ideas for additional coverage:

• Whitespace between and after the padding character =, e.g. Uint8Array.fromBase64("Zg= =") and Uint8Array.fromBase64("Zg= = ").
• Uint8Array.prototype.setFrom{Base64,Hex} doesn't modify the typed array when the input string is invalid.
• Typed array is backed by a SharedArrayBuffer.
• lastChunkHandling option is present, but set to undefined, e.g. the options object is {lastChunkHandling: undefined}.
• lastChunkHandling option is an invalid string value, e.g. the options object is {lastChunkHandling: "bad"}.
• alphabet option is present, but set to undefined, e.g. the options object is {alphabet: undefined}.
• alphabet and lastChunkHandling options are read in the correct order.
• Extra bits with three element trailing chunk and strict chunk handling, e.g. Uint8Array.fromBase64("ZZZ=", {lastChunkHandling: "strict"}). (The two element chunk case is already covered.)

Originally posted by @anba in #3994 (comment)