tests: Add unittest for gpio_ite_it8xxx2_v2

Add a unittest for 100% of the lines in gpio_ite_it8xxx2_v2. The test
fakes the registers by overriding the ECREG macro to call a function
provided by the test.

Does not test the code guarded by
CONFIG_SOC_IT8XXX2_GPIO_GROUP_K_L_DEFAULT_PULL_DOWN

Bug #66401

Signed-off-by: Jeremy Bettis <jbettis@google.com>