Merge rust-bitcoin#5048: fix grammar in API test comments

5cd163d fix grammar in API test comments (viktorking7)

Pull request description:

  ```markdown
  
  Fixed subject-verb agreement in API test documentation comments.
  
  ### Changes
  - **Files affected:** `hashes/tests/api.rs`, `io/tests/api.rs`, `primitives/tests/api.rs`
  - **Fix:** Changed "The point of these tests **are**" → "The point of these tests **is**"
  
  ### Why
  The subject "The point" is singular, requiring the singular verb "is" instead of "are".
  
  ```


ACKs for top commit:
  apoelstra:
    ACK 5cd163d; successfully ran local tests


Tree-SHA512: ad939c64daee5a960574f9625fc4afc21b4e68a1f7ebac1ec9e42196a0428fdc0cf6bf56bb6428f710ec574860ba0b46bc66fff7506ff2fc4c26ba8e8db7f3c2

Author: apoelstra

hashes/tests/api.rs

@@ -2,7 +2,7 @@
 
 //! Test the API surface of `units`.
 //!
-//! The point of these tests are to check the API surface as opposed to test the API functionality.
+//! The point of these tests is to check the API surface as opposed to test the API functionality.
 //!
 //! ref: <https://rust-lang.github.io/api-guidelines/about.html>
 

io/tests/api.rs

@@ -1,6 +1,6 @@
 //! Test the API surface of `io`.
 //!
-//! The point of these tests are to check the API surface as opposed to test the API functionality.
+//! The point of these tests is to check the API surface as opposed to test the API functionality.
 //!
 //! ref: <https://rust-lang.github.io/api-guidelines/about.html>
 

primitives/tests/api.rs

@@ -2,7 +2,7 @@
 
 //! Test the API surface of `primitives`.
 //!
-//! The point of these tests are to check the API surface as opposed to test the API functionality.
+//! The point of these tests is to check the API surface as opposed to test the API functionality.
 //!
 //! ref: <https://rust-lang.github.io/api-guidelines/about.html>