TC1: Does the calculator turn on when teh on button is pressed
TC2: Does itdisplay the number on teh screen that was pressed
TC3: If we enter 2 and then the plus symbol "+" and then
enter2 again,then does it display the answer "4" or not.
TC4: Same way for all the buttons like *,/,-,%, square roots
and all
TC5: When we press the clear button then is the screen
display reduced to "0" or not

in addition to the above,

1- what is the calc output incase of dividing by zero
2- what is the calc output incase of sqrt a negative number 
3- Number.Oper.oper.numer= << for example 1+-3= << it 
should take the last operator
2- test the back space.. should delete last digit but 
incase of (.) it should be deleted with the number to the 
left and input 0 (^_^) for example: (2.1) --> backspace 
gives (2.) then backspace again .. it should give 0 NOT 2