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.
r? @steveklabnik