langref: Explain Zig Test

Updates the Language Reference sections: Comments, Values, and Zig Test.

Zig Test section moved down with the goal "make sure it can be read top to
bottom sensibly" in mind (issue #1524).

Comments and Values section examples changed test declarations to a main
function and expect statement to print statements.

A print statement was added to the "String Literals and Unicode Code Point"
section's example to demonstrate the "u" format specifier.

Zig Test Section:
* Addresses the question: "How does the syntax work?".
* Partially answers the question: "What can I do with the zig test tool?" but
should be sufficient to understand the examples in all of this document.
* Addresses the question: "How does a top-level test block differ from a function definition?"
* Provides a example to run multiple test.

Lacks clear definitions of containers, top-level, order independence, lazy
analysis, resolve, reference.

GitHub Issues: #8221, #8234

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