Don't test doc comments.

rust-lang/rust-bindgen#1384 makes bindgen
respect whitespace in documentation comments instead of trimming them.

This means that some of the comments from the C++ examples try to get compiled
as Rust, with a bad result for `cargo test`s purposes.

Just test using cargo test --lib. An alternative would be to disable doc
comments in bindgen.

Author: emilio

.travis.yml

@@ -32,7 +32,7 @@ before_script:
 script:
   - ccache -z
   - CCACHE=$(which ccache) travis_wait cargo build --verbose $FEATURES
-  - CCACHE=$(which ccache) RUST_BACKTRACE=1 cargo test --verbose $FEATURES
+  - CCACHE=$(which ccache) RUST_BACKTRACE=1 cargo test --lib --verbose $FEATURES
   - ccache -s
 
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