Use std.testing.expect in language reference samples

In this commit, the code samples in the language reference have been changed to
use `std.testing.expect` rather than `std.debug.assert` when they are
written in `test` code. This will teach Zig learners best practices when
they write their own test code.

Not all uses of `std.debug.assert` have been replaced. There are examples where
using `assert` fits the context of the sample.

Using `std.debug.assert` in test code can lead to errors if running tests in
ReleaseFast mode. In ReleaseFast mode, the `unreachable` in `assert` is
undefined behavior. It is possible that `assert` always causes `zig test` to
pass thus possibly leading to incorrect test code outcomes. The goal is to
prevent incorrect code from passing test cases.

Closes #5836

0 files changed, 0 insertions, 0 deletions