[ycabled] increase UT coverage of ycabled daemon (sonic-net#238)

Signed-off-by: vaibhav-dahiya vdahiya@microsoft.com

Description
This PR refactors some of the message handling for CLI transactions' by ycabled for muxcable
Particularly these CLI commands handling is refactored

show mux hwmode mux <port>
show mux hwmode switchmode <port>
config mux hwmode state <port>
config mux hwmode setswitchmode <port>
config mux firmware download/activate/rollback <port>
show mux firmware version <port>
With this PR all these notifications will be covered by Unit-Tests in our code

Motivation and Context
This PR is required to increase the code coverage of ycabled and attain a coverage of > 75%
This PR also removes refactors some code blocks which were not covered by UT

How Has This Been Tested?
Ran the Unit-tests to the corresponding changes
Signed-off-by: vaibhav-dahiya <vdahiya@microsoft.com>