agent: Fix clippy error

```
error: file opened with `create`, but `truncate` behavior not defined
```
`truncate(true)` ensures the file is entirely overwritten with new data
which I believe is the behaviour we want

Signed-off-by: stevenhorsman <steven@uk.ibm.com>

src/agent/src/mount.rs

@@ -537,6 +537,7 @@ mod tests {
 
         OpenOptions::new()
             .create(true)
+            .truncate(true)
             .write(true)
             .open(test_file_filename)
             .expect("failed to create test file");