[diag] Refactor command processing to remove redundant code #8

Job	Run time
channel-manager-csl	3m 7s
expects	6m 17s
packet-verification-1-1-on-1-4	5m 34s
packet-verification-low-power	6m 18s
thread-1-4-gcc-m32	4m 18s
thread-1-4-posix	9m 30s
thread-1-4-gcc-m64	3m 17s
thread-1-4-clang-m32	5m 4s
thread-1-4-clang-m64	10m 3s
upload-coverage	1s
	53m 29s

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