Add googletest::gtest to googletest::prelude

We would like the canonical spelling of the attribute to be `#[gtest]` - short but clearly identifying the library.

PiperOrigin-RevId: 661146513

Author: gribozavr

googletest/src/lib.rs

@@ -42,6 +42,7 @@ pub mod matchers;
 /// }
 /// ```
 pub mod prelude {
+    pub use super::gtest;
     pub use super::matcher::{Matcher, MatcherBase};
     pub use super::matchers::*;
     pub use super::verify_current_test_outcome;