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.

src/doc/trpl/benchmark-tests.md

@@ -13,7 +13,7 @@ pub fn add_two(a: i32) -> i32 {
 }
 
 #[cfg(test)]
-mod tests {
+mod test {
     use super::*;
     use test::Bencher;
 

src/doc/trpl/testing.md

@@ -382,7 +382,7 @@ pub fn add_two(a: i32) -> i32 {
 }
 
 #[cfg(test)]
-mod tests {
+mod test {
     use super::*;
 
     #[test]