[Dev] Add testing

[NobodyXu]

Though it is difficult to perform testing given that the output of swaystatus highly coupled with the current state of the system,
it is still possible to at least some basic form of test for:

• test relative/absoluate configuration loading
• test configuration verification
• test format by setting it to a pure string (not format string)
• test reloading with or without configuration
• testing click events support