Implement formal way to SKIP tests

[kristate]

Instead of blindly passing tests for non-implemented features, it would be nice to be able to skip a test formally.

Once example of such a useful place is tcp.zig:

zig/std/event/tcp.zig

Lines 125 to 129 in b6eb404

	test "listen on a port, send bytes, receive bytes" {
	if (builtin.os != builtin.Os.linux) {
	// TODO build abstractions for other operating systems
	return;
	}

I don't like posting issues without writing some sort of proposal or implementation, so I will spend some time looking at the internals, too. Could possibly ride ontop of the error framework: error.skip ?

[kristate]

Implemented in PR #1275