[FIX] web: tests: patchDate use the offset fixed in luxon

Before this commit, patchDate used the machine offset and not the offset
fixed in luxon.

For example:
    Date to Patch = 01-01-2000 10:30:00
    Machine offset = -120 (UTC-2)
    Luxon offset = 360 (UTC+6)

Before:
    Patched Date = 01-01-2000 10:30:00 UTC-2

After:
    Patched Date = 01-01-2000 02:30:00 UTC-2

closes odoo/odoo#91591

Signed-off-by: Bruno Boi <boi@odoo.com>