tests: drivers: flash: split mixed error conditions in negative tests

Some flash negative tests were validating multiple error conditions
in a single API call, such as combining unaligned offsets with
oversized operations.

This makes it unclear which condition is actually being tested and
can lead to fragile tests if drivers change the order of validation.

Split these cases so that each call checks a single expected error
condition, improving test clarity and robustness.

Signed-off-by: Gaetan Perrot <gaetan.perrot@spacecubics.com>