test(backends): have a variant of `test_decimal_literal` using a decimal literal with explicit scale

[kszucs]

          The ibis datatype of 

ibis.literal(decimal.Decimal("1.1"), type=dt.decimal).type()

is

Decimal(precision=None, scale=None, nullable=True)

so the literal is casted to DECIMAL and MySQL decides about scale and precision.

We should have another test case where we test a decimal literal with explicit scale and precision, and we expect uniform behaviour across all the backends.

Originally posted by @kszucs in #7926 (comment)