vfs: Make tests run reliably

On file Remove
- cancel any writebacks in progress
- ignore error message deleting non existent file if file was in the
  process of being uploaded

Writeback
- Don't transfer the file if it has disappeared in the meantime
- Take our own copy of the file name to avoid deadlocks
- Fix delayed retry logic
- Wait for upload to finish when cancelling upload

Fix race condition in item saving

Fix race condition in vfscache test

Make sure we delete the file on the error path - this makes cascading
failures much less likely

vfs/read_write_test.go

@@ -565,6 +565,11 @@ func TestRWFileHandleSizeCreateNew(t *testing.T) {
 func testRWFileHandleOpenTest(t *testing.T, vfs *VFS, test *openTest) {
 	fileName := "open-test-file"
 
+	// Make sure we delete the file on failure too
+	defer func() {
+		_ = vfs.Remove(fileName)
+	}()
+
 	// first try with file not existing
 	_, err := vfs.Stat(fileName)
 	require.True(t, os.IsNotExist(err))