[FIX] web: tests: fix typos in :{last,first}-child

Follow-up of odoo/enterprise#11881, in order to eliminate all similar
typos.

The CSS pseudo-class selectors are spelled `:first-child` and
`:last-child`, and never take any argument, as opposed to
`:nth-child(<nth>)` for example.

Ref: https://developer.mozilla.org/docs/Web/CSS/Pseudo-classes



jQuery doesn't care and matches with or without the `()`, but CSS
engines don't, and now libsass SCSS compilation crashes due to the empty
argument list (cfr. opw-2299465)

Better avoid confusion and fix the typo everywhere.

closes odoo/odoo#54685

X-original-commit: 82244e17339615615535073e6f9e1ddbcf0b57d9
Signed-off-by: Richard Mathot (rim) <rim@openerp.com>
Signed-off-by: Olivier Dony (odo) <odo@openerp.com>