delete test disk files if they already exist other we'd fail the test (…

…#3)

mayastor/tests/nexus_simple_rebuild.rs

@@ -23,6 +23,7 @@ static NEXUS_SIZE: u64 = 10 * 1024 * 1024;  // 10MiB
 #[test]
 fn rebuild_test() {
 
+    common::delete_file(&[DISKNAME1.into(), DISKNAME2.into()]);
     common::truncate_file(DISKNAME1, NEXUS_SIZE / 1024);
     common::truncate_file(DISKNAME2, NEXUS_SIZE / 1024);