book: use `mod test` consistently

book: use `mod test` consistently

Fixes #24030

Of the four code samples with modules in TRPL:

    - 2 use `mod test`
    - 2 use `mod tests`

We should be consistent here, but which is right? The stdlib is split:

    $ grep -r 'mod tests {' src/lib* | wc -l
          63
    $ grep -r 'mod test {'  src/lib* | wc -l
          58

Subjectively, I like the plural, but both the language reference and the
style guide recommend the singular. So we'll go with that here, for now.