tests/functional/aspeed: Move I2C test into shared helper for AST2700 reuse

tests/functional/aspeed: Move I2C test into shared helper for AST2700 reuse

Move the I2C test case into a common helper function (do_ast2700_i2c_test) so it
can be reused across multiple AST2700-based test cases. This reduces duplication
and improves maintainability.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Reviewed-by: Nabih Estefan <nabihestefan@google.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250423072350.541742-8-jamin_lin@aspeedtech.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>