[diag] Refactor command processing to remove redundant code #9

Job	Run time
channel-manager-csl	3m 2s
expects	32m 38s
packet-verification-1-1-on-1-4	13m 4s
packet-verification-low-power	10m 19s
thread-1-4-gcc-m32	11m 30s
thread-1-4-posix	9m 31s
thread-1-4-gcc-m64	11m 4s
thread-1-4-clang-m32	9m 50s
thread-1-4-clang-m64	9m 44s
upload-coverage	18s
	1h 51m 0s

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