Disable Rust backtraces for tests

Index: tests/testsuite/preprocessor.rs
--- tests/testsuite/preprocessor.rs.orig
+++ tests/testsuite/preprocessor.rs
@@ -4,6 +4,7 @@ use crate::prelude::*;
 use mdbook::book::Book;
 use mdbook::errors::Result;
 use mdbook::preprocess::{CmdPreprocessor, Preprocessor, PreprocessorContext};
+use std::env;
 use std::sync::{Arc, Mutex};
 
 struct Spy(Arc<Mutex<Inner>>);
@@ -56,6 +57,8 @@ fn nop_preprocessor() {
 // Failing preprocessor generates an error.
 #[test]
 fn failing_preprocessor() {
+    env::set_var("RUST_BACKTRACE", "0");
+
     BookTest::from_dir("preprocessor/failing_preprocessor")
         .run("build", |cmd| {
             cmd.expect_failure()
