Add unit tests for Utils::toHex #879

Job	Run time
build (Heltec_v3_room_server)	2m 48s
build (RAK_4631_room_server)	2m 19s
build (Heltec_v3_repeater)	2m 52s
build (RAK_4631_repeater)	2m 24s
build (RAK_4631_companion_radio_ble)	2m 28s
build (wio-e5-mini_repeater)	1m 45s
build (PicoW_repeater)	2m 39s
build (Tbeam_SX1276_repeater)	1m 55s
build (LilyGo_Tlora_C6_repeater_)	2m 2s
build (wio_wm1110_repeater)	2m 20s
build (Heltec_v3_companion_radio_ble)	2m 49s
	26m 21s

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