cache: add coverage for Mount readonly parameter.
cache: add coverage for Mount readonly parameter.

This adds test coverage for ensuring the readonly parameter is honored
as expected in the ref Mount methods. There was a regression introduced
during #2335 that went unnoticed until identified and fixed in #2562.
This test coverage should help prevent similar regressions in the
future.

Signed-off-by: Erik Sipsma <c195b508e6f0c57aeafdb5e96947e2b79e6a4b40@sipsma.dev>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc