In a vignette, put a call to
change_state() before any code block that
makes a change on
the server, or rather, before any code block that might repeat the same
request previously done and expect a different result.
Invisibly, the return of
.mockPaths() with the new path added.
change_state() works by layering a new directory on top of the existing
.mockPaths(), so fixtures are recorded/loaded there, masking rather than
overwriting previously recorded responses for the same request. In
vignettes, these mock layers are subdirectories with integer names.