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

@@ -26,20 +26,14 @@ sort_rand() {
 check_random_corruption() {
   to_break=$(find "$IPFS_PATH/blocks" -type f -name '*.data' | sort_rand | head -n 1)
 
-  test_expect_success "back up file and overwrite it" '
-    cp "$to_break" backup_file &&
-    echo "this is super broken" > "$to_break"
-  '
-
-  test_expect_success "repo verify detects failure" '
+  test_expect_success "repo verify detects a failure" '
+    mv "$to_break" backup_file &&
+    echo -n "this block will not match expected hash" > "$to_break" &&
     test_expect_code 1 ipfs repo verify
   '
 
-  test_expect_success "replace the object" '
-    cp backup_file "$to_break"
-  '
-
-  test_expect_success "ipfs repo verify passes just fine now" '
+  test_expect_success "repo verify passes once a failure is fixed" '
+    mv backup_file "$to_break" &&
     ipfs repo verify
   '
 }