Don't use default build-script fingerprinting in test

This changes the `test` build script so that it does not use the default
fingerprinting mechanism in cargo which causes a full scan of the
package every time it runs. This build script does not depend on any of
the files in the package.

This is the recommended approach for writing build scripts.

Author: ehuss

library/test/build.rs

@@ -1,4 +1,5 @@
 fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rustc-check-cfg=cfg(enable_unstable_features)");
 
     let rustc = std::env::var("RUSTC").unwrap_or_else(|_| "rustc".into());