test: fix flaky repo verify

it seems verify not always notices file was broken, this fix performs
explicit flush and assumes CI uses filesystem with lazy flush or some
other unknown behavior

test/sharness/t0086-repo-verify.sh

@@ -28,15 +28,17 @@ check_random_corruption() {
 
   test_expect_success "back up file and overwrite it" '
     cp "$to_break" backup_file &&
-    echo "this is super broken" > "$to_break"
+    echo "this is super broken" > "$to_break" &&
+    sync "$to_break"
   '
 
   test_expect_success "repo verify detects failure" '
     test_expect_code 1 ipfs repo verify
   '
 
-  test_expect_success "replace the object" '
-    cp backup_file "$to_break"
+  test_expect_success "restore broken object" '
+    cp backup_file "$to_break" &&
+    sync "$to_break"
   '
 
   test_expect_success "ipfs repo verify passes just fine now" '