Answer:   We cannot write test code (test methods) inside of the apex trigger. From API Version 28.0, we cannot write the test methods inside of an apex class which is not decorated with @isTest.We can write test methods only in a class which is decorated with @isTest.

Note: We have a governor limit for the overall Apex Code size of the organization which is of 3 MB. If we decorate a class with @isTest annotation Apex Code Size governor limit will be bypassed.