compiletest: fix typo in runtest.rs

nonexistant -> nonexistent

src/tools/compiletest/src/runtest.rs

@@ -3761,7 +3761,7 @@ impl<'test> TestCx<'test> {
 
     fn delete_file(&self, file: &PathBuf) {
         if !file.exists() {
-            // Deleting a nonexistant file would error.
+            // Deleting a nonexistent file would error.
             return;
         }
         if let Err(e) = fs::remove_file(file) {