TST: add a test on mixed offsets for read_csv
xref #54014

added a test on mixed offsets to increase coverage for `read_csv`
test looks good, but I think `pandas/tests/io/parser/test_parse_dates.py` might be a better location > test looks good, but I think `pandas/tests/io/parser/test_parse_dates.py` might be a better location

Thanks, I moved the test as you suggested. Thank you for reviewing this PR.