cam-tool/test: Use tmpfs when writing to the file system

The test test_deploy_message_a2s is currently opening a file
inside the repository in append mode, this will trigger an error
on read-only file systems and it's wrong because even if the code
doesn't write anything, it should be prevented to write in the
same repository of the test.

To overcome this issue, use the tmp_path_factory mechanism of
Pytest to copy the file under tmpfs and do the operation safely
from there.

Add missing word to the dictionary to please the spell checker.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>