Disable Rust backtraces for tests

Index: tests/testsuite/preprocessor.rs
--- tests/testsuite/preprocessor.rs.orig
+++ tests/testsuite/preprocessor.rs
@@ -6,6 +6,7 @@ use anyhow::Result;
 use mdbook_core::book::{Book, BookItem, Chapter};
 use mdbook_driver::builtin_preprocessors::CmdPreprocessor;
 use mdbook_preprocessor::{Preprocessor, PreprocessorContext};
+use std::env;
 use snapbox::IntoData;
 use std::sync::{Arc, Mutex};
 
@@ -74,6 +75,8 @@ fn nop_preprocessor() {
 // Failing preprocessor generates an error.
 #[test]
 fn failing_preprocessor() {
+    unsafe { env::set_var("RUST_BACKTRACE", "0"); }
+
     BookTest::from_dir("preprocessor/failing_preprocessor").run("build", |cmd| {
         cmd.expect_failure()
             .expect_stdout(str![[""]])
