Avoid File::create_new for compatibility with project MSRV

Besides the style change done in the previous commit, `clippy` also
identified `File::create_new` is newer than we may wish to rely on:

  error: current MSRV (Minimum Supported Rust Version) is `1.76.0` but this item is stable since `1.77.0`
     --> tests/tools/src/lib.rs:906:13
      |
  906 |             File::create_new(path)
      |             ^^^^^^^^^^^^^^^^

This changes that code to use `OpenOptions` instead. This is
simlar to the suggested equivalent code in the `File::create_new`
documentation:

  File::options().read(true).write(true).create_new(true).open(...)

But the approach used here differs in that `read(true)` is removed,
since this call is only creating and writing, not reading.

Author: EliahKagan

tests/tools/src/lib.rs

@@ -903,7 +903,10 @@ mod tests {
 
         // Create the files.
         for path in paths {
-            File::create_new(path)
+            File::options()
+                .write(true)
+                .create_new(true)
+                .open(path)
                 .expect("can create file")
                 .write_all(CONFIG_DATA)
                 .expect("can write contents");