No possible to mark default vader temp test file as modified

Given the vader test below, if found out that it is not possible to mark the vader temporary test file as modified.

```vader
Given (whatever):
Bear claw gummi bears brownie cake. Sesame snaps cotton candy powder macaroon.

Do (Mark file as modified):
:let &mod = 1\
:let &ro = 0\

Then (whatever):
Log 'modified: ' . &mod
Log 'readonly: ' . &ro

Expect (whatever):
Bear claw gummi bears brownie cake. Sesame snaps cotton candy powder macaroon.
```

results will be:
```vim
modified: 0
readonly: 0
```
Any idea how to work around that?
It this case [Given/Do/Then/Expect] does Vader create a temp file behind the scene?