Use specific locale for cmp_fast_path test

The test was failing in the regular MacOS terminal due to it defaulting
to LC_ALL=C. Best to standardize like the other tests that check for
locale-dependent output.

tests/integration.rs

@@ -864,6 +864,7 @@ mod cmp {
         let mut cmd = cargo_bin_cmd!("diffutils");
         cmd.arg("cmp");
         cmd.arg(&a_path).arg(&b_path);
+        cmd.env("LC_ALL", "en_US");
         cmd.assert()
             .code(predicate::eq(1))
             .failure()