[I2C] Code clean up for i2c_host_tx_rx_test.c

[vrozic]

Description

Per discussion in #16972 the code for the i2c host test can be improved by:

• Utilizing function i2c_testutils_connect_i2c_to_pinmux_pins
• Using anonymous enum instead of a magic number
• Using an array of structs to simplify switch statements

[johngt]

Suggest that this goes to P4 as a clean-up task. If this needs to be included then will need to raise priority back up to P2 and add an effort estimate.