compiletest: ensure extra arguments are appended at the end (#3379)

We must not mix arguments to be passed to Kani with those parsed by
compiletest. Will enable use of `--cbmc-args` to increase CBMC's
verbosity as needed for certain tests.

tools/compiletest/src/runtest.rs

@@ -272,14 +272,14 @@ impl<'test> TestCx<'test> {
  .arg("kani")
  .arg("--target-dir")
  .arg(self.output_base_dir().join("target"))
- .current_dir(parent_dir)
- .args(&self.config.extra_args);
+ .current_dir(parent_dir);
  if test {
  cargo.arg("--tests");
  }
  if "expected" != self.testpaths.file.file_name().unwrap() {
  cargo.args(["--harness", function_name]);
  }
+ cargo.args(&self.config.extra_args);
 
  let proc_res = self.compose_and_run(cargo);
  self.verify_output(&proc_res, &self.testpaths.file);