Hai,

test cases for bulb 
check the bulb is req shap and size
Check the bulb is fitted and removed from holder
check the bulb glow req illumunation r not
check the bulb  it should glow when we swicth on 
check the bulb it should off when we switch off
check the bulb material

dear sujatha u gave nice answer but still i want to add 
some points.

Test cases for bulb 
First check the fillament of the bulb
Then check the bulb having  required shap,size,power rating 
     ,candella(illumination)and it's holder.
Then check the required power source is available or not.
(The rated voltage available or not like 230V if it is not 
available testing of bulb is incomplete)
Then check the illumination by powering up light.
The bulb should glow immediately after powering up.
The bulb should off immediately after power off.
Check the bulb durability by switching on and off for 
some 5 times.

if you still have any doubt mail me:  
rajendra_penumalli@yahoo.com
or call me:9885162742(Hyd)

testcase for Bulb
1)verify bulb is made for glass or not
2)verify the how many candles for blub
3)verify it is fitted to the holder or not 
4)verisy it is working or not 
5)check after fitting the bulb it is working or not

If any mistakes is thr please excuse