add documentation for zig test

[andrewrk]

extracted from #367

• basic usage
• example of how lazy analysis means you have to reference a declaration for it to get analyzed. so tests are pretty useful. you'll also need to sometimes do comptime { _ = @import("foo.zig"); } to find tests from other files
• --test-filter
• --test-name-prefix
• testing in releasefast and releasesafe mode. use assertOrPanic. std.debug.assert should not be special-cased in test mode #1304
• it puts the test binary in zig-cache/ so you can use a debugger or other tool on it.